#include <bits/stdc++.h>
inline int min(const int &first, const int &second) {
  return (((second - first) >> (32 - 1)) & (first ^ second)) ^ first;
}
inline int max(const int &first, const int &second) {
  return (((second - first) >> (32 - 1)) & (first ^ second)) ^ second;
}
inline long long min(const long long &first, const long long &second) {
  return (((second - first) >> (64 - 1)) & (first ^ second)) ^ first;
}
inline long long max(const long long &first, const long long &second) {
  return (((second - first) >> (64 - 1)) & (first ^ second)) ^ second;
}
using namespace std;
char string_in_buffer[(int)260];
void fast_scan(int &first) { scanf("%d", &first); }
void fast_scan(long long &first) { scanf("%lld", &first); }
void fast_scan(unsigned long long &first) { scanf("%llu", &first); }
void fast_scan(double &first) { scanf("%lf", &first); }
void fast_scan(long double &first) { scanf("%Lf", &first); }
void fast_scan(char &first) {
  scanf("%c", &first);
  if (first == '\n') {
    fast_scan(first);
  }
}
void fast_scan(string &first) {
  scanf("%s", string_in_buffer);
  first = string(string_in_buffer);
}
template <class TFirst, class TSecond>
void fast_scan(pair<TFirst, TSecond> &p) {
  fast_scan(p.first);
  fast_scan(p.second);
}
template <class T>
void fast_scan(vector<T> &v) {
  for (auto &first : v) fast_scan(first);
}
void fast_print(const int &first) { printf("%d", first); }
void fast_print(const unsigned int &first) { printf("%u", first); }
void fast_print(const long long &first) { printf("%lld", first); }
void fast_print(const unsigned long long &first) { printf("%llu", first); }
void fast_print(const double &first) { printf("%.15lf", first); }
void fast_print(const long double &first) { printf("%.15Lf", first); }
void fast_print(const char &first) { printf("%c", first); };
void fast_print(const string &first) { printf("%s", first.c_str()); }
void fast_print(const char v[]) { fast_print((string)v); }
template <class TFirst, class TSecond>
void fast_print(const pair<TFirst, TSecond> &p) {
  fast_print(p.first);
  fast_print(' ');
  fast_print(p.second);
}
template <class T>
void fast_print(const vector<T> &v) {
  if (v.empty()) return;
  fast_print(v[0]);
  for (int i = 1; i < v.size(); i++) {
    fast_print(' ');
    fast_print(v[i]);
  }
}
template <class T>
void fast_print(const vector<vector<T>> &v) {
  if (v.empty()) return;
  fast_print(v[0]);
  for (int i = 1; i < v.size(); i++) {
    fast_print('\n');
    fast_print(v[i]);
  }
}
template <class T>
void fast_print(const T &v) {
  for (const auto &first : v) {
    fast_print(first);
    fast_print(' ');
  }
}
using namespace std;
namespace smart_io {
string print_start = "";
string sep = " ";
bool first_print = false;
void precall_print() {
  fast_print(print_start);
  print_start = "\n";
  first_print = true;
}
void _print(deque<string>) {}
template <class T, class... Args>
void _print(deque<string> names, T elem, Args... args) {
  if (!first_print) {
    fast_print("\n");
  } else {
    first_print = false;
  }
  fast_print(names.front());
  fast_print(" = ");
  fast_print(elem);
  names.pop_front();
  _print(names, args...);
}
}  // namespace smart_io
template <class T>
ostream &operator,(ostream &os, const T &object) {
  if (!smart_io::first_print) {
    fast_print(smart_io::sep);
  } else {
    smart_io::first_print = false;
  }
  fast_print(object);
  return os;
}
template <class T>
istream &operator,(istream &is, T &object) {
  fast_scan(object);
  return is;
}
namespace escape__from__random__aetuhoetnuhshe {
using namespace std::chrono;
mt19937 rng(duration_cast<milliseconds>(system_clock::now().time_since_epoch())
                .count());
uniform_real_distribution<> prob_dist(0.0, 1.0);
};  // namespace escape__from__random__aetuhoetnuhshe
namespace typedefs {}
namespace numbers_operation {
template <class T>
T floor_mod(T a, T b) {
  if (a >= 0 && b >= 0) return a % b;
  if (a <= 0 && b <= 0) return a % b;
  return abs(b) - (abs(a) % abs(b));
}
}  // namespace numbers_operation
using namespace numbers_operation;
using namespace typedefs;
using namespace escape__from__random__aetuhoetnuhshe;
vector<pair<int, vector<pair<int, int>>>> graphs = {
    make_pair(3, vector<pair<int, int>>{make_pair(0, 1), make_pair(1, 2),
                                        make_pair(0, 2)}),
    make_pair(4, vector<pair<int, int>>{make_pair(0, 1), make_pair(1, 2),
                                        make_pair(2, 3), make_pair(3, 0)}),
    make_pair(4, vector<pair<int, int>>{make_pair(0, 1), make_pair(1, 2),
                                        make_pair(2, 3), make_pair(3, 0),
                                        make_pair(0, 2)}),
    make_pair(5, vector<pair<int, int>>{make_pair(0, 1), make_pair(1, 2),
                                        make_pair(2, 3), make_pair(3, 4),
                                        make_pair(4, 0)}),
    make_pair(5, vector<pair<int, int>>{make_pair(0, 1), make_pair(1, 2),
                                        make_pair(2, 3), make_pair(3, 4),
                                        make_pair(4, 0), make_pair(0, 2)}),
    make_pair(5, vector<pair<int, int>>{make_pair(0, 1), make_pair(1, 2),
                                        make_pair(2, 3), make_pair(3, 4),
                                        make_pair(4, 0), make_pair(0, 2),
                                        make_pair(1, 3)}),
    make_pair(6, vector<pair<int, int>>{make_pair(0, 1), make_pair(1, 2),
                                        make_pair(2, 3), make_pair(3, 4),
                                        make_pair(4, 5), make_pair(5, 0)}),
    make_pair(6, vector<pair<int, int>>{make_pair(0, 1), make_pair(1, 2),
                                        make_pair(2, 3), make_pair(3, 4),
                                        make_pair(4, 5), make_pair(5, 0),
                                        make_pair(0, 2)}),
    make_pair(6, vector<pair<int, int>>{make_pair(0, 1), make_pair(1, 2),
                                        make_pair(2, 3), make_pair(3, 4),
                                        make_pair(4, 5), make_pair(5, 0),
                                        make_pair(0, 2), make_pair(1, 3)}),
    make_pair(8, vector<pair<int, int>>{
                     make_pair(0, 1), make_pair(1, 2), make_pair(2, 3),
                     make_pair(3, 4), make_pair(4, 5), make_pair(5, 6),
                     make_pair(6, 7), make_pair(7, 0), make_pair(0, 2),
                     make_pair(1, 3), make_pair(4, 6)})};
const int N = 4000;
char mem[1010][4010];
bool dp(int n, int cur) {
  if (n < 0) return false;
  if (mem[n][cur] != -1) return mem[n][cur];
  if (n == 0) {
    bool prime = true;
    for (int j = 2; j * j <= cur; j++) {
      if (cur % j == 0) {
        prime = false;
        break;
      }
    }
    return prime;
  }
  if (n <= 2) return false;
  for (auto g : graphs) {
    if (dp(n - g.first, cur + ((int)g.second.size()))) {
      return mem[n][cur] = true;
    }
  }
  return mem[n][cur] = false;
}
vector<pair<int, int>> edges;
void go(int n, int cur, int offset) {
  if (n == 0) return;
  for (auto g : graphs) {
    if (dp(n - g.first, cur + ((int)g.second.size()))) {
      for (pair<int, int> p : g.second)
        edges.emplace_back(p.first + offset, p.second + offset);
      go(n - g.first, cur + ((int)g.second.size()), offset + g.first);
      return;
    }
  }
}
signed main(signed argc, char *argv[]) {
  memset(mem, -1, sizeof mem);
  int n;
  cin, n;
  go(n, 0, 1);
  smart_io::precall_print();
  cout, ((int)edges.size());
  for (pair<int, int> p : edges) {
    smart_io::precall_print();
    cout, p;
  }
}
