#include <bits/stdc++.h>
using namespace std;
long long int prime[] = {
    2,   3,   5,   7,   11,  13,  17,  19,  23,  29,  31,  37,  41,  43,  47,
    53,  59,  61,  67,  71,  73,  79,  83,  89,  97,  101, 103, 107, 109, 113,
    127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197,
    199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281,
    283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379,
    383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463,
    467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557, 563, 569, 571,
    577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647, 653, 659,
    661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757, 761,
    769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863,
    877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977,
    983, 991, 997, 1009};
long long int dp[1005][1005];
long long int degree[1005];
void solve() {
  long long int n;
  cin >> n;
  long long int num = lower_bound(prime, prime + 169, n) - prime;
  long long int diff = prime[num] - n;
  for (long long int i = 1; i <= n - 1; i++) {
    dp[i][(i + 1)] = 1;
    dp[i + 1][(i)] = 1;
  }
  dp[1][n] = 1;
  dp[n][1] = 1;
  for (long long int i = 1; i <= n; i++) {
    degree[i] = 2;
  }
  set<pair<long long int, long long int> > ans;
  for (long long int d = 1; d <= diff; d++) {
    for (long long int i = 1; i <= n; i++) {
      for (long long int j = i + 1; j <= n; j++) {
        if (dp[i][j] == 0 && dp[j][i] == 0 && degree[i] == 2 &&
            degree[j] == 2) {
          dp[i][j] = dp[j][i] = 1;
          degree[i]++, degree[j]++;
          goto xy;
        }
      }
    }
  xy:;
  }
  for (long long int i = 1; i <= n; i++) {
    for (long long int j = i; j <= n; j++) {
      if (dp[i][j]) ans.insert({i, j});
    }
  }
  cout << (long long int)ans.size();
  cout << "\n";
  for (auto i : ans) {
    cout << i.first << ' ' << i.second;
    cout << "\n";
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  {
    solve();
    cout << "\n";
  }
}
