#include using namespace std; using ll = long long; using ld = long double; template using V = vector; using VI = V; using VL = V; using VS = V; template using PQ = priority_queue, greater>; using graph = V; template using w_graph = V>>; #define FOR(i,a,n) for(int i=(a);i<(n);++i) #define eFOR(i,a,n) for(int i=(a);i<=(n);++i) #define rFOR(i,a,n) for(int i=(n)-1;i>=(a);--i) #define erFOR(i,a,n) for(int i=(n);i>=(a);--i) #define all(a) a.begin(),a.end() #define rall(a) a.rbegin(),a.rend() #define out(y,x) ((y)<0||h<=(y)||(x)<0||w<=(x)) #ifdef _DEBUG #define line cout << "-----------------------------\n" #define stop system("pause") #endif constexpr ll INF = 1000000000; constexpr ll LLINF = 1LL << 60; constexpr ll mod = 1000000007; constexpr ll MOD = 998244353; constexpr ld eps = 1e-10; templateinline bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; }return false; } templateinline bool chmin(T& a, const T& b) { if (a > b) { a = b; return true; }return false; } inline void init() { cin.tie(nullptr); cout.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } templateinline istream& operator>>(istream& is, V& v) { for (auto& a : v)is >> a; return is; } templateinline istream& operator>>(istream& is, pair& p) { is >> p.first >> p.second; return is; } templateinline V vec(size_t a) { return V(a); } templateinline V defvec(T def, size_t a) { return V(a, def); } templateinline auto vec(size_t a, Ts... ts) { return V(ts...))>(a, vec(ts...)); } templateinline auto defvec(T def, size_t a, Ts... ts) { return V(def, ts...))>(a, defvec(def, ts...)); } templateinline void print(const T& a) { cout << a << "\n"; } templateinline void print(const T& a, const Ts&... ts) { cout << a << " "; print(ts...); } templateinline void print(const V& v) { for (int i = 0; i < v.size(); ++i)cout << v[i] << (i == v.size() - 1 ? "\n" : " "); } templateinline void print(const V>& v) { for (auto& a : v)print(a); } templateinline T sum(const V& a, int l, int r) { return a[r] - (l == 0 ? 0 : a[l - 1]); } templateinline void END(T s) { print(s); exit(0); } void END() { exit(0); } int main() { init(); int n; cin >> n; int k = n - 1, r = n; V> ans; eFOR(i, 2, n) { ans.emplace_back(i - 1, i); //print(i - 1, i); --k; for (int j = 2; j < i && k && r >i; --k, ++j) { //print(i, r); ans.emplace_back(i, r); r -= 1; } if (r <= i)break; } print(ans.size()); for (auto [x, y] : ans)print(x, y); return 0; }