#include <bits/stdc++.h>
using namespace std;
long long int myXOR(long long int x, long long int y) {
  return (x | y) & (~x | ~y);
}
long long int powerLL(long long int x, long long int n) {
  long long int result = 1;
  while (n) {
    if (n & 1) result = result * x % 1000000007;
    n = n / 2;
    x = x * x % 1000000007;
  }
  return result;
}
bool comp(long long int a, long long int b) { return a > b; }
bool prime[1000001];
void seive() {
  long long int p, i;
  for (i = 1; i < 1000001 + 1; i++) prime[i] = true;
  prime[1] = false;
  for (p = 2; p * p <= 1000001; p++) {
    if (prime[p] == true) {
      for (i = p * p; i <= 1000001; i += p) {
        prime[i] = false;
      }
    }
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long int t, n, m, i, j, k, l, r, x, y, a, b, c, d, e, f, p, nl, np, q;
  string str, str1;
  cin >> n;
  seive();
  for (i = 1; i <= 1000; i++) {
    if (prime[i * n + 1] == false) {
      cout << i;
      return 0;
    }
  }
}
