#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef pair<ll,ll> P;
typedef vector<ll> VI;
typedef vector<VI> VVI;
#define REP(i,n) for(ll i=0;i<(n);i++)
#define ALL(v) v.begin(),v.end()
constexpr ll MOD=1000000007;
constexpr ll INF=2e18;

int main(){
    int n; cin >> n;
    vector<P> ans;
    VI c(n,0);
    REP(i,n){
        for(int j=i+1;j<n;j++){
            if(c[i]>=n-i-1)
                break;
            ans.push_back({i,j});
            c[i]++, c[j]++;
        }
    }
    cout << ans.size() << endl;
    for(auto p:ans)
        cout << p.first+1 << " " << p.second+1 << endl;
}