#define _USE_MATH_DEFINES
#define _SILENCE_CXX17_C_HEADER_DEPRECATION_WARNING
#define _SILENCE_ALL_CXX17_DEPRECATION_WARNINGS
#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
#include<iostream>
#include<unordered_set>

using namespace std;

#define dprint(Exp,...) if(Exp){fprintf(stderr, __VA_ARGS__);}
#define printe(...) fprintf(stderr, __VA_ARGS__);
//#define printe(...) void();
#define printE(...) fprintf(stderr, __VA_ARGS__);
//#define printE(...) void();


#define PrtExp(_Exp)  cerr<< #_Exp <<" = "<< (_Exp)
#define PrtExpN(_Exp)  cerr<< #_Exp <<" = "<< (_Exp) <<"\n"

#define SINT(n) scanf("%d",&n)
#define SINT2(n,m) scanf("%d %d",&n,&m)
#define SINT3(n,m,o) scanf("%d %d %d",&n,&m,&o)
#define SINT4(n,m,o,P) scanf("%d %d %d %d",&n,&m,&o,&P)
#define SINT5(n,m,o,P,q) scanf("%d %d %d %d %d",&n,&m,&o,&P,&q)
#define SLL(n) scanf("%lld",&n)
#define SLL2(n,m) scanf("%lld %lld",&n,&m)
#define SLL3(n,m,o) scanf("%lld %lld %lld",&n,&m,&o)
#define SST(s) scanf("%s",s)
#define SCH(c) scanf("%c",&c)

#define GC() getchar()

#define PINT(n) printf("%d",(int)(n))
#define PINT2(n,m) printf("%d %d",(int)(n),(int)(m))
#define PINT3(n,m,l) printf("%d %d %d",(int)(n),(int)(m),(int)(l))
#define PLL(n) printf("%lld",(long long)(n))
#define PST(s) printf("%s",(s))
#define PCH(s) printf("%c",(s))

#define PINTN(n) printf("%d\n",(int)(n))
#define PINT2N(n,m) printf("%d %d\n",(int)(n),(int)(m))
#define PINT3N(n,m,l) printf("%d %d %d\n",(int)(n),(int)(m),(int)(l))
#define PLLN(n) printf("%lld\n",(long long)(n))
#define PSTN(s) printf("%s\n",(s))
#define PCHN(s) printf("%c\n",(s))

#define PSP() printf(" ")
#define PN() printf("\n")

#define PC(c) putchar(c)
#define CSP (' ')
#define SN ("\n")

#define rep(i,a) for(int i=0;i<a;i++)
#define reP(i,a) for(int i=0;i<=a;i++)
#define Rep(i,a) for(int i=a-1;i>=0;i--)
#define ReP(i,a) for(int i=a;i>=0;i--)

#define rEp(i,a) for(i=0;i<a;i++)
#define rEP(i,a) for(i=0;i<=a;i++)
#define REp(i,a) for(i=a-1;i>=0;i--)
#define REP(i,a) for(i=a;i>=0;i--)

#define repft(i,a,b) for(int i=a;i<b;i++)
#define repfT(i,a,b) for(int i=a;i<=b;i++)
#define Repft(i,a,b) for(int i=a-1;i>=b;i--)
#define RepfT(i,a,b) for(int i=a;i>=b;i--)

#define foreach(a,it) for(auto it = a.begin(); it != a.end(); ++it)

#define FILL(a,v) fill(begin(a),end(a), v)
#define FILL0(a) memset(a,0,sizeof(a))
#define FILL1(a) memset(a,-1,sizeof(a))

typedef long long ll;

typedef unsigned long long ull;
typedef pair<int, int> Pi;
typedef pair<ll, ll>   Pll;


const int INF = 1'010'000'000; // 0x3C33'6080
const ll INFLL = 0x1f1f1f1f1f1f1f1fLL;//2,242,545,357,980,376,863

template <class A, class B> inline ostream& operator<<(ostream& st, const pair<A, B>& P) { return st << "(" << P.first << "," << P.second << ")"; };
template <class A, class B> inline pair<A, B> operator+(const pair<A, B>& P, const pair<A, B>& Q) { return pair<A, B>(P.first + Q.first, P.second + Q.second); };
template <class A, class B> inline pair<A, B> operator-(const pair<A, B>& P, const pair<A, B>& Q) { return pair<A, B>(P.first - Q.first, P.second - Q.second); };

#define fs  first
#define sc  second

int bitCount(unsigned int bits) {
	bits = (bits & 0x55555555) + (bits >> 1 & 0x55555555);    //  2bitごとに計算
	bits = (bits & 0x33333333) + (bits >> 2 & 0x33333333);    //  4bitごとに計算
	bits = (bits & 0x0f0f0f0f) + (bits >> 4 & 0x0f0f0f0f);    //  8bitごとに計算
	bits = (bits & 0x00ff00ff) + (bits >> 8 & 0x00ff00ff);    //  16ビットごとに計算   
	return (bits & 0x0000ffff) + (bits >> 16);    //  32ビット分を計算   
}


struct uf_tree {
	std::vector<int> parent;
	int __size;
	uf_tree(int size_) : parent(size_, -1), __size(size_) {}
	void unite(int x, int y) {
		if ((x = find(x)) != (y = find(y))) {
			if (parent[y] < parent[x]) std::swap(x, y);
			parent[x] += parent[y];
			parent[y] = x;
			__size--;
		}
	}
	bool is_same(int x, int y) { return find(x) == find(y); }
	int find(int x) { return parent[x] < 0 ? x : parent[x] = find(parent[x]); }
	int size(int x) { return -parent[find(x)]; }
	int size() { return __size; }
};


bool connect[16][16];

void dfs(int i, int j, int n) {
	// cout << i << " " << j << endl;
	/*
	rep(i, n) {
		rep(j, n) {
			cout << connect[i][j] << " ";
		}
		cout << endl;
	}
	cout << endl;
	*/

	if (i == n -1 ) {
		uf_tree U(n);
		rep(i, n) repft(j, i + 1, n) {
			if (connect[i][j]) U.unite(i, j);
		}
		if (U.size(0) != n) return;
		int S = 0;
		rep(i, n) if (connect[0][i]) S += i + 1;

		repft(j, 1, n) {
			int s = 0;
			rep(i, n) if (connect[j][i]) s += i + 1;
			if (S != s) return;
		}

		puts("FOUND!");
		rep(i, n) repft(j, i + 1, n) {
			if (connect[i][j]) {
				cout << i + 1 << " " << j + 1 << endl;
			}
		}
		rep(i, n) {
			rep(j, n) {
				cout << (connect[i][j] ? "#,": " ,");
			}
			cout << endl;
		}
		cout << endl;
		rep(i, n) {
			rep(j, n) {
				cout << (connect[i][j] ? " ," : "X,");
			}
			cout << endl;
		}


		cout << endl;
		return;
	}
	int I = i;
	int J = j + 1;
	if (J == n) {
		++I;
		J = I + 1;
	}
	connect[j][i] = connect[i][j] = true;
	dfs(I, J, n);
	connect[j][i] = connect[i][j] = false;
	dfs(I, J, n);
}

void solve(const int n) {
	dfs(0, 0, n);

	cout << "FIN" << endl;
}


int main() {
	/*
	for (;;) {
		int i;
		cin >> i;
		solve(i);
	}
	*/
	int n;
	cin >> n;
	vector<Pi> ret;
	if (n % 2 == 0) {
		rep(i, n) repft(j, i + 1, n) {
			if (i + j == n - 1) continue;
			ret.push_back({i, j});
		}
	} else {
		rep(i, n) repft(j, i + 1, n) {
			if (i + j == n - 2) continue;
			ret.push_back({ i, j });
		}
	}
	cout << ret.size() << endl;
	for (auto&p : ret) {
		cout << p.first+1 << " " << p.second+1 << endl;
	}
}




Measured memory_bytes: 384