#include <bits/stdc++.h>
//typedef
//-------------------------#include <bits/stdc++.h>
 
const double pi = 3.141592653589793238462643383279;
 
 
using namespace std;
 
//conversion
//------------------------------------------
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }
inline int readInt() { int x; scanf("%d", &x); return x; }
 
//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
 
 
//container util
 
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a)*(a))
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
 
 
//repetition
//------------------------------------------
#define FOR(i,s,n) for(int i=s;i<(int)n;++i)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007
 
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()
 
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
const double EPS = 1E-8;
 
#define chmin(x,y) x=min(x,y)
#define chmax(x,y) x=max(x,y)
 
class UnionFind {
public:
    vector <int> par; 
    vector <int> siz; 

    UnionFind(int sz_): par(sz_), siz(sz_, 1) {
        for (ll i = 0; i < sz_; ++i) par[i] = i;
    }
    void init(int sz_) {
        par.resize(sz_);
        siz.assign(sz_, 1LL);
        for (ll i = 0; i < sz_; ++i) par[i] = i;
    }
 
    int root(int x) { 
        while (par[x] != x) {
            x = par[x] = par[par[x]];
        }
        return x;
    }
 
    bool merge(int x, int y) {
        x = root(x);
        y = root(y);
        if (x == y) return false;
        if (siz[x] < siz[y]) swap(x, y);
        siz[x] += siz[y];
        par[y] = x;
        return true;
    }
 
    bool issame(int x, int y) { 
        return root(x) == root(y);
    }
 
    int size(int x) { 
        return siz[root(x)];
    }
};
 
class WeightedUnionFind{
public:
    vector <int> par; 
    vector <int> siz;
    vector <ll> diff_weight; /* 頂点間の重みの差 */

    WeightedUnionFind(int sz_): par(sz_), siz(sz_, 1LL), diff_weight(sz_, 0LL){
        for(int i=0; i<sz_; i++) par[i] = i;
    }

    void init(int sz_){
        par.resize(sz_);
        siz.assign(sz_, 1LL);
        diff_weight.resize(sz_);
        for(int i=0; i<sz_; i++) par[i] = i, diff_weight[i] = 0;
    }

    int root(int x){
        if(par[x] == x){
            return x;
        }else{
            int r = root(par[x]);
            diff_weight[x] += diff_weight[par[x]];
            return par[x] = r;
        }
    }

    ll weight(ll x){
        root(x);
        return diff_weight[x];
    }

    bool issame(int x, int y){
        return root(x) == root(y);
    }

    bool merge(int x, int y, ll w){
        w += weight(x); w -= weight(y);
        x = root(x); y = root(y);
        if(x == y) return false;

        if (siz[x] < siz[y]) swap(x, y), w = -w;
        siz[x] += siz[y];
        par[y] = x;
        diff_weight[y] = w;
        return true;
    }

    ll diff(int x, int y){
        return weight(y) - weight(x);
    }

};
 
ll modPow(ll x, ll n, ll mod = MOD){
    if(n <= 0) return 1;
    ll res = 1;
    while(n){
        if(n&1) res = (res * x)%mod;
 
        res %= mod;
        x = x * x %mod;
        n >>= 1;
    }
    return res;
}
 
#define SIEVE_SIZE 500000+10
bool sieve[SIEVE_SIZE];
void makeSieve(){
    for(int i=0; i<SIEVE_SIZE; ++i) sieve[i] = true;
    sieve[0] = sieve[1] = false;
    for(int i=2; i*i<SIEVE_SIZE; ++i) if(sieve[i]) for(int j=2; i*j<SIEVE_SIZE; ++j) sieve[i*j] = false;
}
 
bool isprime(ll n){
    if(n == 0 || n == 1) return false;
    for(ll i=2; i*i<=n; ++i) if(n%i==0) return false;
    return true;
}
 
const int MAX = 2000010;
long long fac[MAX], finv[MAX], inv[MAX];
 
// テーブルを作る前処理
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;
}
 
long long extGCD(long long a, long long b, long long &x, long long &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    long long d = extGCD(b, a%b, y, x);
    y -= a/b * x;
    return d;
}
// 負の数にも対応した mod (a = -11 とかでも OK) 
inline long long mod(long long a, long long m) {
    return (a % m + m) % m;
}
 
// 逆元計算 (ここでは a と m が互いに素であることが必要)
long long modinv(long long a, long long m) {
    long long x, y;
    extGCD(a, m, x, y);
    return mod(x, m); // 気持ち的には x % m だが、x が負かもしれないので
}
ll GCD(ll a, ll b){
    
    if(b == 0) return a;
    return GCD(b, a%b);
}
 

template< typename Monoid, typename OperatorMonoid = Monoid >
struct LazySegmentTree
{
  using F = function< Monoid(Monoid, Monoid) >;
  using G = function< Monoid(Monoid, OperatorMonoid) >;
  using H = function< OperatorMonoid(OperatorMonoid, OperatorMonoid) >;
  using P = function< OperatorMonoid(OperatorMonoid, int) >;

  int sz;
  vector< Monoid > data;
  vector< OperatorMonoid > lazy;
  const F f;
  const G g;
  const H h;
  const P p;
  const Monoid M1;
  const OperatorMonoid OM0;


  LazySegmentTree(int n, const F f, const G g, const H h, const P p,
                  const Monoid &M1, const OperatorMonoid OM0)
      : f(f), g(g), h(h), p(p), M1(M1), OM0(OM0)
  {
    sz = 1;
    while(sz < n) sz <<= 1;
    data.assign(2 * sz, M1);
    lazy.assign(2 * sz, OM0);
  }

  void set(int k, const Monoid &x)
  {
    data[k + sz] = x;
  }

  void build()
  {
    for(int k = sz - 1; k > 0; k--) {
      data[k] = f(data[2 * k + 0], data[2 * k + 1]);
      data[k] %= MOD;
    }
  }

  void propagate(int k, int len)
  {
    if(lazy[k] != OM0) {
      if(k < sz) {
        lazy[2 * k + 0] = h(lazy[2 * k + 0], lazy[k]);
        lazy[2 * k + 1] = h(lazy[2 * k + 1], lazy[k]);
      }
      data[k] = g(data[k], p(lazy[k], len));
      lazy[k] = OM0;
    }
  }

  Monoid update(int a, int b, const OperatorMonoid &x, int k, int l, int r)
  {
    propagate(k, r - l);
    if(r <= a || b <= l) {
      return data[k];
    } else if(a <= l && r <= b) {
      lazy[k] = h(lazy[k], x);
      propagate(k, r - l);
      return data[k];
    } else {
      return data[k] = f(update(a, b, x, 2 * k + 0, l, (l + r) >> 1),
                         update(a, b, x, 2 * k + 1, (l + r) >> 1, r));
    }
  }

  Monoid update(int a, int b, const OperatorMonoid &x)
  {
    return update(a, b, x, 1, 0, sz);
  }


  Monoid query(int a, int b, int k, int l, int r)
  {
    propagate(k, r - l);
    if(r <= a || b <= l) {
      return M1;
    } else if(a <= l && r <= b) {
      return data[k];
    } else {
      return f(query(a, b, 2 * k + 0, l, (l + r) >> 1),
               query(a, b, 2 * k + 1, (l + r) >> 1, r));
    }
  }

  Monoid query(int a, int b)
  {
    return query(a, b, 1, 0, sz);
  }

  Monoid operator[](const int &k)
  {
    return query(k, k + 1);
  }
};

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    //cout << fixed << setprecision(15);

    int N; cin >> N;

    vector<pair<int, int>> vp;
    if(N%2 == 1){
      int mx = N*(N+1)/2 - N;
      for(int i=1; i<=N; i++){

        int tmp = N*(N+1)/2-i;
        for(int j=i+1; j<=N; j++){
          if(tmp-mx == j) continue;
          vp.push_back({i, j});
        }
      }
      cout << vp.size() << endl;
      for(int i=0; i<vp.size(); i++){
        cout << vp[i].first << " " << vp[i].second << endl;
      }
    }else{
      int mx = N*(N+1)/2 - (N+1);
      for(int i=1; i<=N; i++){

        int tmp = N*(N+1)/2-i;
        for(int j=i+1; j<=N; j++){
          if(tmp-mx == j) continue;
          vp.push_back({i, j});
        }
      }
      cout << vp.size() << endl;
      for(int i=0; i<vp.size(); i++){
        cout << vp[i].first << " " << vp[i].second << endl;
      }
    }
    
    return 0;
}

Measured memory_bytes: 384