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#include <bits/stdc++.h>
using namespace std;

const int MAXN = 105;

vector<int> edges[MAXN];
int names[MAXN], rev_names[MAXN];
int N = 0;

pair<int,int> gen(int n) {
  if (n == 1) {
    edges[N+1].push_back(N+2);
    N += 2;
    return make_pair(N-1, N);
  }
  if (n % 2 == 0) {
    pair<int,int> ab = gen(n / 2);
    edges[ab.second].push_back(N+1);
    edges[ab.second].push_back(N+2);
    edges[N+1].push_back(N+2);
    N += 2;
    return make_pair(ab.first, N);
  }
  else {
    pair<int,int> ab = gen(n - 1);
    edges[N+1].push_back(ab.first);
    edges[N+1].push_back(ab.second);
    N++;
    return make_pair(N, ab.second);
  }
}

int main() {
  int n;
  cin >> n;
  pair<int,int> ab = gen(n);

  for (int i = 1; i <= N; i++) names[i] = i;

  while (true) {
    random_shuffle(names + 1, names + N + 1);
    if (names[ab.first] != 1) continue;
    if (names[ab.second] != N) continue;
    break;
  }
  for (int i = 1; i <= N; i++)
    rev_names[names[i]] = i;

  cout << N << "\n";
  for (int i = 1; i <= N; i++) {
    int u = rev_names[i];
    if (edges[u].size() >= 1) cout << names[edges[u][0]];
    else cout << "-1";
    cout << " ";
    if (edges[u].size() >= 2) cout << names[edges[u][1]];
    else cout << "-1";
    cout << "\n";
    assert(edges[u].size() <= 2);
  }


  return 0;
}