#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using u64 = unsigned long long;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;

int main() {
    int n;
    cin >> n;
    if(n == 2) puts("0");
    else if(n == 3) puts("1\n2 3");
    if(n <= 3) return 0;
    vector<pair<int, int>> v;
    v.emplace_back(1, 2);
    for (int i = 4; i <= n; ++i) {
        if(i%2 == 0){
            for (int j = 0; j < i-1; ++j) {
                v.emplace_back(j, i-1);
            }
        }
    }
    cout << v.size() << "\n";
    for (auto &&[a, b] : v) printf("%d %d\n", a+1, b+1);
    return 0;
}