#include <bits/stdc++.h>
using namespace std;
bool SortbySecDesc(const pair<long long, long long> &a,
                   const pair<long long, long long> &b) {
  return a.second > b.second;
}
bool sortinrev(const pair<int, int> &a, const pair<int, int> &b) {
  return (a.first > b.first);
}
long long Prime[3000000], nPrime;
long long mark[10000002];
void seive(long long int n) {
  long long int i, j, limit = sqrt(n * 1.0) + 2;
  mark[1] = 1;
  for (i = 4; i <= n; i += 2) mark[i] = 1;
  Prime[nPrime++] = 2;
  for (i = 3; i <= n; i += 2) {
    if (!mark[i]) {
      Prime[nPrime++] = i;
      if (i <= limit) {
        for (j = i * i; j <= n; j += i * 2) {
          mark[j] = 1;
        }
      }
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(nullptr);
  ;
  long long n;
  cin >> n;
  seive(10000);
  for (int i = 1; i <= 1000; i++) {
    long long ans = n * i + 1;
    if (mark[ans] == 1) {
      cout << i << "\n";
      ;
      return 0;
    }
  }
  return 0;
}
