#include <iostream>
#include <algorithm>

using namespace std;

const int N_PRIMEVEC = 1000000;
bool primevec[N_PRIMEVEC];

void setupPrimes()
{
  fill(primevec, primevec + N_PRIMEVEC, true);

  primevec[0] = false;
  primevec[1] = false;

  for (int i = 2; i < N_PRIMEVEC; ++i)
    if (primevec[i])
      for (int j = i+i; j < N_PRIMEVEC; j += i)
	primevec[j] = false;

  return;
}

int main()
{
  setupPrimes();

  while (true) {
    int a, d, n;
    cin >> a >> d >> n;
    if (a == 0 && d == 0 && n == 0) break;

    int count = 0;
    int ans = -1;
    for (int x = a; ; x += d) {
      if (primevec[x]) ++count;

      if (n == count) {
	ans = x;
	break;
      }
    }

    cout << ans << '\n';
  }

  return 0;
}