#include <bits/stdc++.h>   using namespace std;   int prime(int n) {     if (n == 1) {       return 0;     }     for (int i = 2; i * i <= n; i++) {       if (n % i == 0) {         return 0;       }     }     return 1;   }   int main() {     int n;     cin >> n;     int cnt = n;     while (!prime(cnt)) {       cnt++;     }     cout << cnt << endl;     for (int i = 1; i <= n; i++) {       cout << i <<     << i % n + 1 << endl;     }     int now = 1;     while (cnt != n) {       cout << now <<     << n - now << endl;       now++;       cnt--;     }     return 0;   }   