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#include "bits/stdc++.h"
using namespace std;
template <class A, class B> ostream& operator<<(ostream &os, pair<A, B> p) {
  return os << '{' << p.first << ", " << p.second << '}';
}
template <class T> ostream& operator<<(ostream &os, vector<T> v) {
  os << '{';
  for (T i : v) os << i << ", ";
  return os << '}';
}
#ifdef DEBUG
#define D(x...) x
#else
#define D(x...)
#endif
#define LN(x) D(cerr << #x << ": " << x << ' ')
#define LOG(x) D(cerr << #x << ": " << x << '\n')
#define MES(x) D(cerr << x << ' ')
#define MESL(x) D(cerr << x << '\n')
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<bool> vb;
typedef pair<int, int> pii;
#define EB emplace_back
#define PB pop_back
#define fi first
#define se second
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define REP(i, n) FOR(i, 0, (n) - 1)

// Ignacy Boehlke

int main() {
    int n;
    scanf("%d", &n);
    int N = 2, n2 = n;
    vector<pii> graph(N, {-1, -1});
    while (n2 > 1) {
        graph.resize(N += 2, {-1, -1});
        graph[N - 3] = {N - 1, N - 2};
        graph[N - 2].fi = N - 1;
        n2 >>= 1;
    }
    graph.back().fi = N;
    graph.EB(-1, -1);
    REP(i, N / 2 - 1) if (n & (1 << i)) graph[i * 2 + 2].se = N;

    printf("%d\n", N);
    FOR(i, 1, N) printf("%d %d\n", graph[i].fi, graph[i].se);
}