#include <iostream>
#include <string>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <list>
#include <iterator>
#include <cassert>
#include <numeric>
#include <functional>
#include <ctime>
#pragma warning(disable:4996) 

//#define ATCODER
#ifdef ATCODER
#include <atcoder/all>
#endif

typedef long long ll;
typedef unsigned long long ull;
#define LINF  9223300000000000000
#define LINF2 1223300000000000000
#define LINF3 1000000000000
#define INF 2140000000
const long long MOD = 1000000007;
//const long long MOD = 998244353;

using namespace std;
#ifdef ATCODER
using namespace atcoder;
#endif


void solve()
{
    int n;
    scanf("%d", &n);

    vector<pair<int, int>> v;
    for (int i = 0; i < n; i++) {
        for (int j = i + i + 1; j < n; j++) {
            v.push_back(make_pair(i, j));
        }
    }
    printf("%d\n", (int)v.size());
    for (int i = 0; i < (int)v.size(); i++) {
        printf("%d %d\n", v[i].first + 1, v[i].second + 1);
    }

    return;
}

int main()
{
#if 1
    solve();
#else
    int T, t;
    scanf("%d", &T);
    for (t = 0; t < T; t++) {
        //printf("Case #%d: ", t + 1);
        solve();
    }
#endif
    return 0;
}