Prime numbers detector

#include <iostream>
#include <math.h>

using namespace std;

void prime_numbers_check(int a){
    int counter = 0;
    for(int i = 2; i <= (int)sqrt(a); i ++)
        if(a % i == 0) counter ++;
    if(counter > 0) cout << a << " is a composite number" << endl;
    else cout << a << " is a prime number" << endl;
}

int main()
{
    int a;
    bool flag = 1;
    while (flag){
        cout << "Enter integer number" << endl;
        cin >> a;
        prime_numbers_check(a);
        cout << "Press: 1 - continue, 0 - finish ";
        cin >> flag;
        cout << endl;
    }
    return 0;
}