#include <iostream>
#include <cmath>

using namespace std;

int N(int N0, float t, float t12)
{
    return N0 * pow(.5, (t/t12));
}

int main()
{
    float t12;
    int N0;
    float t;

    cout << "What's the half-life of your isotope?: ";
    cin >> t12;
    cout << "What's the initial population N0?: ";
    cin >> N0;
    cout << "What time t do you want to evaluate N(t)? (use same unit of time): ";
    cin >> t;

    cout << "The population at time " << t << " is " << N(N0, t, t12);
    return 0;
}