Code unexpectadly giving an infinite loop - c++

Question

Recently started learning C++ and i wrote a simple program that defines is the given number prime or not, and when the input is zero, it finishes. But i accidentally noticed that it creates an infinite loop when the input is somewhere between 2100000000 and 2200000000. I don't know why this happens, can you please explain to me?
#include <iostream>
using namespace std;
int number, i, k;
int main()
{
do
{
cin >> number;
if (number == 0)
break;
for (i = 2; i < round(sqrt(number)) + 1; i++)
if (number % i == 0)
k++;
if ((number == 1) || (k > 0))
cout << "This number is not prime\n";
else
cout << "This number is prime\n";
k = 0;
i = 0;
} while (!(number == 0));
}

Answer 1

The value overflows its type (int) which maximum value is 2147483647 and it results in Undefined Behaviour. If you need to accept large numbers as input you may have to use a long long. Additionally, why check if number equals 0 twice?
#include <iostream>
#include <math.h>
int main()
{
while(true)
{
long long number;
bool isPrime = true;
std::cin >> number;
if(number == 0)
break;
for (int i = 2; i < round(sqrt(number)) + 1; i++)
{
if (number % i == 0)
{
isPrime = false;
break;
}
}
if (isPrime)
std::cout << "This number is prime\n"; //<== print this is this case
else
std::cout << "This number is not prime\n";
}
return 0;
}