How Can I Create A Timer Based User Entry Input?

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C++

My goal is that i will ask a user a question and say i will provide 10 seconds to answer that question.If the user exits the given time then the program terminates. But even though the time has been exceeded, i cant seem to terminate the program. Basically what i mean is that the program still keeps waiting for the user's input.
I cant understand how to fix this error. Please any suggestions! Thanks

#include<iostream>
#include <ctime>
#include<stdlib.h>
#include <conio.h>
#include <string>
#include <Windows.h>

using namespace std;

int main()
{
time_t begin,end;
bool flag = true;
string ch;

begin = time(NULL);
while(flag)
{
  end = time(NULL);
  
  if(difftime(end,begin)<30)
   {
    cout<<"Enter your answer"<<endl;
    getline(cin,ch);
   }

  if(difftime(end,begin)>30)
   {
    flag = false;
    cout<<"Sorry you are timed out"<<endl;
   }
}

system("pause");
}