#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e7 + 5;
int f[maxn], sum[maxn], a, b, k;
void init() {
  f[0] = 1;
  f[1] = 1;
  for (int i = 2; i <= 1000000; i++) {
    if (f[i] == 0) {
      for (int j = i + i; j <= 1000000; j += i) f[j] = 1;
    }
  }
  sum[0] = sum[1] = 0;
  for (int i = 2; i <= 1000000; i++) {
    if (f[i] == 0)
      sum[i] = sum[i - 1] + 1;
    else
      sum[i] = sum[i - 1];
  }
}
int fun(int x) {
  for (int i = a; i <= b - x + 1; i++)
    if (sum[i + x - 1] - sum[i - 1] < k) {
      return 0;
    }
  return 1;
}
int main() {
  init();
  scanf("%d %d %d", &a, &b, &k);
  if (sum[b] - sum[a - 1] < k)
    printf("-1\n");
  else {
    int start = 1, end = b - a + 1, ans = 0;
    while (start <= end) {
      int mid = (start + end) / 2;
      if (fun(mid)) {
        ans = mid;
        end = mid - 1;
      } else
        start = mid + 1;
    }
    if (ans == 0)
      puts("-1");
    else
      printf("%d\n", ans);
  }
}
