Pi Calculation

#include <iostream>
#include <math.h>
using namespace std;
double calculate(double epsilon) {
    double last = sqrt(0.5);
    double current = sqrt(0.5 + (0.5 * last));
    double result = last * current;
    while (abs(current - last) >= epsilon) {
        last = current;
        current = sqrt(0.5 + (0.5 * current));
        result = result * current;
    }
    return result;
}
int main()
{
    double epsilon;
    cin >> epsilon;
    cout << 4.0/calculate(epsilon);
    system("pause");
}