#include <bits/stdc++.h>
using namespace std;
bool prime[3005];
template <typename A, typename B>
string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string& s) { return '"' + s + '"'; }
string to_string(const char* s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto& x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cout << '\n'; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cout << " " << to_string(H);
  debug_out(T...);
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n, x, t, m, y, z, k;
  memset(prime, true, sizeof(prime));
  for (int p = 2; p * p <= 3000; p++) {
    if (prime[p] == true) {
      for (int i = p * p; i <= 3000; i += p) prime[i] = false;
    }
  }
  cin >> n;
  vector<pair<long long, long long> > ans;
  for (int i = 0; i < n; i++) {
    ans.push_back({i, (i + 1) % (n)});
  }
  long long sum = ans.size();
  for (int i = 0; i < n - 2; i++) {
    if (i % 4 > 1) continue;
    if (prime[sum]) break;
    sum++;
    ans.push_back({i, (i + 2) % n});
  }
  assert(prime[sum]);
  cout << sum << '\n';
  for (auto it : ans) {
    cout << it.first + 1 << " " << it.second + 1 << '\n';
  }
  return 0;
}
