#include<bits/stdc++.h>
    using namespace std;
    
    //long long
    using ll = long long;
    
    // pair<int, int>
    using PII = pair<int, int>;
    
    
    
    //最大値、mod
    const int MOD = 1000000007;
    const int mod = 1000000007;
    const int INF = 1000000001;
    const long long LINF = 1e18;
    const int MAX = 510000;
    
    //出力系
    #define print(x) cout << x << endl
    #define prints(x) cout << fixed << setprecision(10) << x << endl
    #define printc(x) cout << setw(2) << setfill('0') << x << endl;
    #define yes cout << "Yes" << endl
    #define YES cout << "YES" << endl
    #define no cout << "No" << endl
    #define NO cout << "NO" << endl
    
    // begin() end()
    #define all(x) (x).begin(),(x).end()
    
    //for
    #define REP(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
    
    //最大公約数 
    unsigned gcd(unsigned a, unsigned b) {
    if(a < b) return gcd(b, a);
    unsigned r;
    while ((r=a%b)) {
        a = b;
        b = r;
    }
    return b;
    }
    
    // 最小公倍数
    unsigned lcm(unsigned a, unsigned b){
        return a / gcd(a, b) * b; 
    }
    
    // a = max(a, b), a = min(a, b)
    template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
    template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
    
    // 階乗(MODをとる)
    ll pow_mod(ll num, ll pow, ll mod) {
        ll prod = 1;
        num %= mod;
        while (pow > 0) {
            if (pow & 1) prod = prod * num % mod;
            num = num * num % mod;
            pow >>= 1;
        }
        return prod;
    }
    
    
    
    // 二項係数(MODとる、1 ≦ k ≦ n ≦ 10^7 程度)
    // COMinit()
    // COM(x, y)
    // とコンビで使う
    
    // テーブルを作る前処理
    long long fac[MAX], finv[MAX], inv[MAX];
    void COMinit() {
        fac[0] = fac[1] = 1;
        finv[0] = finv[1] = 1;
        inv[1] = 1;
        for (int i = 2; i < MAX; i++){
            fac[i] = fac[i - 1] * i % MOD;
            inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
            finv[i] = finv[i - 1] * inv[i] % MOD;
        }
    }
    
    // 二項係数計算
    long long COM(int n, int k){
        if (n < k) return 0;
        if (n < 0 || k < 0) return 0;
        return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
    }
    
    int dx[4] = {0, 0, 1, -1}, dy[4] = {1, -1, 0, 0};
    
    
    
    struct UnionFind {
        vector<int> par;
        vector<int> rank;
        vector<ll> Size;
        UnionFind(int n = 1) {
            init(n);
        }
    
        void init(int n = 1) {
            par.resize(n + 1); rank.resize(n + 1); Size.resize(n + 1);
            for (int i = 0; i <= n; ++i) par[i] = i, rank[i] = 0, Size[i] = 1;
        }
    
        int root(int x) {
            if (par[x] == x) {
                return x;
            }
            else {
                int r = root(par[x]);
                return par[x] = r;
            }
        }
    
        bool issame(int x, int y) {
            return root(x) == root(y);
        }
    
        bool merge(int x, int y) {
            x = root(x); y = root(y);
            if (x == y) return false;
            if (rank[x] < rank[y]) swap(x, y);
            if (rank[x] == rank[y]) ++rank[x];
            par[y] = x;
            Size[x] += Size[y];
            return true;
        }
    
        ll size(int x){
            return Size[root(x)];
        }
    };
    #define rrep(i,a,b) for(int i=(a);i>(b);i--)
    #define rep(i,a,b) for(int i=(a);i<(b);i++)
    
    
    struct Mint {
        int val;
        Mint inv() const{
            int tmp,a=val,b=mod,x=1,y=0;
            while(b)tmp=a/b,a-=tmp*b,swap(a,b),x-=tmp*y,swap(x,y);
            return Mint(x);
        }
    public:
        Mint():val(0){}
        Mint(ll x){if((val=x%mod)<0)val+=mod;}
        Mint pow(ll t){Mint res=1,b=*this; while(t){if(t&1)res*=b;b*=b;t>>=1;}return res;}
        Mint& operator+=(const Mint& x){if((val+=x.val)>=mod)val-=mod;return *this;}
        Mint& operator-=(const Mint& x){if((val+=mod-x.val)>=mod)val-=mod; return *this;}
        Mint& operator*=(const Mint& x){val=(ll)val*x.val%mod; return *this;}
        Mint& operator/=(const Mint& x){return *this*=x.inv();}
        bool operator==(const Mint& x) const{return val==x.val;}
        bool operator!=(const Mint& x) const{return val!=x.val;}
        bool operator<(const Mint& x) const{return val<x.val;}
        bool operator<=(const Mint& x) const{return val<=x.val;}
        bool operator>(const Mint& x) const{return val>x.val;}
        bool operator>=(const Mint& x) const{return val>=x.val;}
        Mint operator+(const Mint& x) const{return Mint(*this)+=x;}
        Mint operator-(const Mint& x) const{return Mint(*this)-=x;}
        Mint operator*(const Mint& x) const{return Mint(*this)*=x;}
        Mint operator/(const Mint& x) const{return Mint(*this)/=x;}
    };
    struct factorial {
        vector<Mint> Fact, Finv;
    public:
        factorial(int maxx){
            Fact.resize(maxx+1),Finv.resize(maxx+1); Fact[0]=Mint(1); rep(i,0,maxx)Fact[i+1]=Fact[i]*(i+1);
            Finv[maxx]=Mint(1)/Fact[maxx]; rrep(i,maxx,0)Finv[i-1]=Finv[i]*i;
        }
        Mint fact(int n,bool inv=0){if(inv)return Finv[n];else return Fact[n];}
        Mint nPr(int n,int r){if(n<0||n<r||r<0)return Mint(0);else return Fact[n]*Finv[n-r];}
        Mint nCr(int n,int r){if(n<0||n<r||r<0)return Mint(0);else return Fact[n]*Finv[r]*Finv[n-r];}
    };

    ll modfact(ll n) { 
        if (n <= 1) return 1;
        return (n * modfact(n - 1)) % MOD;
    }



    int main(){
        int n; cin >> n;
        cout << n * (n - 1) / 2 - n / 2 << endl;
          for(int i = 2;i <= n; i++){
            for(int j = 1; j < i; j++){
            if(i + j != n + 1- n % 2) cout << j << ' ' << i << endl;
                }
            }
  		return 0;
    }


Measured memory_bytes: 256