generated apk suddenly crashes after installation

package com.vis.vis;

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;


public class VIS extends AppCompatActivity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.content_vis);
        final Button btn = (Button)findViewById(R.id.button1);
        final EditText et1 = (EditText)findViewById(R.id.editText1);
        final EditText et2 = (EditText)findViewById(R.id.editText2);
        final EditText et3 = (EditText)findViewById(R.id.editText3);
        final EditText et4 = (EditText)findViewById(R.id.editText4);
        final TextView result = (TextView)findViewById(R.id.textView1);
        btn.setOnClickListener(new OnClickListener()
        {

            @Override
            public void onClick(View v)
            {
                int s1 = new Integer(et1.getText().toString());
                int s2 = new Integer(et2.getText().toString());
                int s3 = new Integer(et3.getText().toString());
                float a = new Integer(et4.getText().toString());
                float end;
                end = (float) (((s1  + s2  + s3 ) / 7.5) + (a / 10));
                if(end<=40)
                {
                    end = (40 - end)*2;
                    result.setText("The marks needed to pass in your end semester:  " +end);
                }
                else
                {
                    end = (end - 40)/2;
                   result.setText("Additional marks that you have :"+end);
                }

            }
        });

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "visweswaran.nagasivam98@gmail.com", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_vi, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }


    }

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto" android:layout_width="match_parent"
    android:layout_height="wrap_content" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    app:layout_behavior="@string/appbar_scrolling_view_behavior" tools:showIn="@layout/activity_vis"
    tools:context=".VIS">

    <TextView android:text="Kalasalingam University Mark Calculator" android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView android:text="\n\nProgrammed by Er.N.Visweswaran.,B.Tech.,DMO.,DCP.," android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView android:text="\n\nEnter your sessional one marks:  " android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <EditText
        android:id="@+id/editText1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="textPersonName" >

        <requestFocus />
    </EditText>
    <TextView android:text="\n\nEnter your sessional two marks:  " android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <EditText
        android:id="@+id/editText2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="textPersonName" />
    <TextView android:text="\n\nEnter your sessional three marks:  " android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <EditText
        android:id="@+id/editText3"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="textPersonName" />
    <TextView android:text="\n\nEnter your assignment marks:  " android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <EditText
        android:id="@+id/editText4"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="textPersonName" />
    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="CALCULATE" />
    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Large Text"
        android:textAppearance="?android:attr/textAppearanceLarge" />



</RelativeLayout>