#include <bits/stdc++.h>
using namespace std;
double PI = acos(-1.0);
long long mod = 1e9 + 7;
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }
  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
long long GCD(long long a, long long b) {
  if (b == 0) return a;
  return GCD(b, a % b);
}
long long prod(long long a, long long b) {
  long long res = 0;
  a %= mod;
  while (b) {
    if (b & 1ll) res = (res + a) % mod;
    a = (2ll * a) % mod;
    b >>= 1ll;
  }
  return res;
}
long long power(long long a, long long b) {
  long long res = 1ll;
  while (b > 0) {
    if (b % 2ll) res = (res * a) % mod;
    a = (a * a) % mod;
    b /= 2ll;
  }
  return res;
}
long long binaryExponentiation(long long a, long long b) {
  long long result = 1ll;
  while (b > 0) {
    if (b % 2 == 1) result = (result * a) % mod;
    a = (a * a) % mod;
    b /= 2ll;
  }
  return result;
}
long long ModularInverse(long long a) {
  return binaryExponentiation(a, mod - 2);
}
void solve() {
  long long n;
  cin >> n;
  if (n <= 2) {
    cout << -1 << "\n";
    return;
  }
  vector<bool> prime(1010, true);
  prime[1] = false;
  for (long long i = 2; i * i <= 1009; i++) {
    if (!prime[i]) continue;
    for (long long j = i * i; j <= 1009; j += i) {
      prime[j] = false;
    }
  }
  long long x = n;
  while (!prime[x]) x++;
  long long r = x - n;
  vector<pair<long long, long long>> ans;
  for (long long i = 1; i < n; i++) {
    ans.push_back({i, i + 1});
  }
  ans.push_back({n, 1});
  long long j = n / 2 + 1;
  for (long long i = 1; i <= n / 2 and r > 0; i++) {
    ans.push_back({i, j});
    j++;
    r--;
  }
  cout << ans.size() << "\n";
  for (auto &e : ans) {
    cout << e.first << " " << e.second << "\n";
  }
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long t = 1;
  while (t--) {
    solve();
  }
}
