//
//
//  Codeforces: ganariya
//  AtCoder: ganariya2525
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//  MMMMMMMMMMMMMMMMNOtXtdMMMMMNxzZMMNmgHyVVVVVW%..?/(.,(x-..........(HfffpffpffpffWHHffpffppfVyyWMMMMMM


//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>

using namespace std;

#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v){std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}

using LL = long long;

//~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~

int main() {

    int N;
    cin >> N;

    vector<pair<int, int>> ans;
    for (int i = 1; i <= N; i++) {
        for (int j = i + 1; j <= N; j++) {
            ans.push_back(make_pair(i, j));
        }
    }

    if (N % 2 == 0) {
        for (int i = 1; i <= N / 2; i++) {
            int j = (N + 1) - i;
            ans.erase(find(ans.begin(), ans.end(), make_pair(i, j)));
        }
    } else {
        for (int i = 1; i <= N / 2; i++) {
            int j = N - i;
            ans.erase(find(ans.begin(), ans.end(), make_pair(i, j)));
        }
    }

    cout << ans.size() << endl;
    for (int i = 0; i < ans.size(); i++) {
        cout << ans[i].first << " " << ans[i].second << endl;
    }

    return 0;
}










































Measured memory_bytes: 384