#include <bits/stdc++.h>
using namespace std;
const int N = 1000010;
int cnt[N];
int a, b, n;
void genPrime() {
  for (int i = 2; i < N; ++i) cnt[i] = 1;
  for (int i = 2; i <= 1000; ++i)
    if (cnt[i])
      for (int j = i * i; j < N; cnt[j] = 0, j += i)
        ;
  for (int i = 1; i < N; ++i) cnt[i] += cnt[i - 1];
}
bool chk(int x) {
  for (int i = a; i + x - 1 <= b; ++i) {
    int j = i + x - 1;
    if (cnt[j] - cnt[i - 1] < n) return false;
  }
  return true;
}
int main() {
  int i, j, k;
  genPrime();
  scanf("%d%d%d", &a, &b, &n);
  if (cnt[b] - cnt[a - 1] < n) {
    printf("-1\n");
    return 0;
  }
  int low = 1, up = b - a + 1;
  while (up > low) {
    int mid = (up + low) / 2;
    if (chk(mid))
      up = mid;
    else
      low = mid + 1;
  }
  printf("%d\n", up);
  return 0;
}
