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#include <cstdio>
#include <cstring>
#include <cmath>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <iterator>
#include <string>
#include <vector>
#include <queue>
#include <bitset>
#include <utility>
#include <stack>
#include <set>
#include <map>

using namespace std;
typedef long long LL;
typedef pair<int,int> PII;
typedef vector<int> VI;
#define MP make_pair
#define FOR(v,p,k) for(int v=(p);v<=(k);++v)
#define FORD(v,p,k) for(int v=(p);v>=(k);--v)
#define REP(i,n) for(int i=0;i<(n);++i)
#define VAR(v,i) __typeof(i) v=(i)
#define FOREACH(i,c) for(VAR(i,(c).begin());i!=(c).end();++i)
#define PB push_back
#define ST first
#define ND second
#define SIZE(x) (int)x.size()
#define ALL(c) c.begin(),c.end()

#define ODD(x) ((x)%2)
#define EVEN(x) (!(ODD(x)))

int debug = 0;
int n=0;
map<int, VI> deg_to_node_ids;
map<int, int>  node_to_deg;
void dfs(int k, int cnt) {
    if (k <= 1) {
        return;
    }
    auto &t = deg_to_node_ids[k];
    bool no_need_recursive = !t.empty();
    int to_add = cnt - t.size();
    REP(i, to_add) {
        t.PB(++n);
    }
    if (no_need_recursive) {
        return;
    }

    int left = k/2;
    int right = k - left;
    assert(left <= right);
    if (left == right) {
        dfs(left, 2);
    } else {
        dfs(left, 1);
        dfs(right, 1);
    }
}

void print_node(int from) {
    int k = node_to_deg[from];
    int left_id, right_id;
    if (k == 0) {
        left_id = -1;
        right_id = -1;
    } else if (k == 1) {
        left_id = n;
        right_id = -1;
    } else {
        int left = k/2;
        int right = k-left;
        assert(left <= right);
        if (left == 1) {
            left_id = n;
            right_id = deg_to_node_ids[right][0];
        } else if (left == right) {
            left_id = deg_to_node_ids[left][0];
            right_id = deg_to_node_ids[right][1];
        } else {
            left_id = deg_to_node_ids[left][0];
            right_id = deg_to_node_ids[right][0];
        }
    }
    printf("%d %d\n", left_id, right_id);
}

int main() {
    int k;
    scanf("%d", &k);
    dfs(k, 1);
    deg_to_node_ids[1].PB(++n);
    deg_to_node_ids[0].PB(++n);
    assert(n <= 100);
    for (auto const& [key, val] : deg_to_node_ids) {
        assert(val.size() <= 2);
        REP(i, val.size()) {
            node_to_deg[val[i]] = key;
            if (debug) {
                printf("node_id: %d deg: %d\n", val[i], key);
            }
        }
    }
    printf("%d\n", n);
    FOR(from, 1, n) {
        print_node(from);
    }
    return 0;
}