#include <bits/stdc++.h>
using namespace std;
int const N = 1e6 + 4;
int a, b, k, cs[N];
bool prime[N];
void sieve() {
  memset(prime, true, sizeof prime);
  prime[0] = prime[1] = false;
  for (int i = 2; i * i < N; ++i)
    if (prime[i])
      for (int j = i * i; j < N; j += i) prime[j] = false;
  cs[0] = 0;
  for (int i = 1; i < N; ++i) cs[i] = cs[i - 1] + prime[i - 1];
}
bool can(int mid) {
  for (int i = a; i <= b - mid + 1; ++i)
    if (cs[i + mid] - cs[i] < k) return false;
  return true;
}
int main() {
  sieve();
  scanf("%d %d %d", &a, &b, &k);
  int l = 1, r = b - a + 1, mid, res = -1;
  while (l <= r) {
    mid = (l + r) >> 1;
    if (can(mid)) {
      r = mid - 1;
      res = mid;
    } else
      l = mid + 1;
  }
  printf("%d\n", res);
  return 0;
}
