POJ3006 - Dirichlet's Theorem on Arithmetic Progressions

#include <iostream>
#include <math.h>
using namespace std;

int main()
{
    int N = 1000000;
    bool prime[N+1];
    for (int i = 0; i < N; i++)
    {
        if (i%2 == 0 && i != 2)
            prime[i] = false;
        else
            prime[i] = true;
    }
    prime[1] = false;
    for (int i = 3; i <= sqrt(N); i += 2)
    {
        if (prime[i])
            for (int j = i+i; j <= N; j += i)
                prime[j] = false;
    }

    int a, d, n;
    int seq;
    int nth;
    while (cin >> a >> d >> n)
    {
        seq = a;
        nth = 1;
        if (a == 0 && d == 0 && n == 0)
            return 0;

        while (1)
        {
            if (prime[seq] && nth == n)
                break;
            if (prime[seq])
                nth++;
            seq += d;
        }
        
        cout << seq << endl;
    }

    return 0;
}