#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#include <array>
#include <list>
#include <atcoder/all>
#define popcount __builtin_popcount
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef pair<int, int> P;

int main()
{
    int n;
    cin>>n;
    int l=1, r=n;
    vector<P> ans;
    for(; l<r; l++, r--){
        for(int j=l; j<r; j++){
            ans.push_back({j, r});
        }
    }
    cout<<ans.size()<<endl;
    for(auto p:ans) cout<<p.first<<" "<<p.second<<endl;
    return 0;
}