#include <bits/stdc++.h>
using namespace std;
bool sortinrev(long long x, long long y) { return x > y; }
void swap(long long* a, long long* b) {
  long long temp = *a;
  *a = *b;
  *b = temp;
}
long long div_ceil(long long a, long long b) { return (a + b - 1) / b; }
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long lcm(long long a, long long b) { return (a / gcd(a, b)) * b; }
const int MOD = 1000 * 1000 * 1000 + 7;
const long long INF = 1e18L + 5;
long long fpow(long long a, long long b) {
  if (b == 0) return 1;
  if (b % 2 == 1) {
    return fpow(a, b - 1) * a % MOD;
  } else {
    long long b2 = fpow(a, b / 2) % MOD;
    return b2 * b2 % MOD;
  }
}
const int nax = 200005;
const int N = int(1e6);
bool isPrime(long long n) {
  if (n <= 1) return false;
  if (n <= 3) return true;
  if (n % 2 == 0 || n % 3 == 0) return false;
  for (long long i = 5; i * i <= n; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0) return false;
  return true;
}
void solve() {
  long long n;
  cin >> n;
  long long m = n;
  while (!isPrime(m)) m++;
  cout << m << "\n1 " << n << "\n";
  for (long long i = 0; i < n - 1; i++) cout << i + 1 << " " << i + 2 << "\n";
  for (long long i = 0; i < m - n; i++)
    cout << i + 1 << " " << i + 1 + n / 2 << "\n";
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  long long T = 1;
  while (t--) {
    cerr << "TEST-CASE #" << T << "\n";
    solve();
    T++;
    cerr << "\n";
  }
  cerr << "\nI am Winning";
  cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
}
