How to check for enter key while scanning input

// Write a program to calculate the addition of  integers and print the output.As we  //are taking integer as input from user we don't know exact number of inputs  he/she is //going to press.

// Input Format:
// line 1: Integers delimited by space

// sample input: 3 4 5 6 7
// sapmle output: 25

// sample input: 3 5 a b 7
//sapmle output: Invalid Input

// Now check what i have tried you will get real problem what i meant to say

#include<iostream>
#include<string>
#include<cstdlib>
using namespace std;
int main()
{
  string temp;
  bool enter = false;
  bool bad_ip = false;
  int sum = 0;
  int a = 0;
  
  
  while(!enter)
  {
      getline(cin,temp); 
      
      if((cin >> a).fail())
      {
         bad_ip = true;
         cout << "Invalid Input" << endl;
         cin.clear();
         exit(1);
      }
      sum += a;
      if(temp.empty())  // i can't compare with newline or enterkey
      {
        cout << sum;
        enter = true;
        break;
      }
      
  }
 
 return 0;
}