#include<vector>
#include<set>
#include<map>
#include<queue>
#include<string>
#include<algorithm>
#include<iostream>
#include<bitset>
#include<functional>
#include<numeric>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cassert>
#include<cmath>
#include<iomanip>
#include<random>
#include<ctime>
#include<complex>
using namespace std;
typedef long long LL;
typedef double D;
#define all(v) (v).begin(), (v).end()
mt19937 gene(233);
typedef complex<double> Complex;
#define fi first
#define se second
#define ins insert
#define pb push_back
inline char GET_CHAR(){
    const int maxn = 131072;
    static char buf[maxn],*p1=buf,*p2=buf;
    return p1==p2&&(p2=(p1=buf)+fread(buf,1,maxn,stdin),p1==p2)?EOF:*p1++;
}
inline int getInt() {
    int res(0);
    char c = getchar();
    while(c < '0') c = getchar();
    while(c >= '0') {
        res = res * 10 + (c - '0');
        c = getchar();
    }
    return res;
}

inline LL fastpo(LL x, LL n, LL mod) {
    LL res(1);
    while(n) {
        if(n & 1) {
            res = res * (LL)x % mod;
        }
        x = x * (LL) x % mod;
        n /= 2;
    }
    return res;
}
LL gcd(LL a, LL b) { return b ? gcd(b, a % b) : a; }

inline string itoa(int x, int width = 0) {
    string res;
    if(x == 0) res.push_back('0');
    while(x) {
        res.push_back('0' + x % 10);
        x /= 10;
    }
    while((int)res.size() < width) res.push_back('0');
    reverse(res.begin(), res.end());
    return res;
}
const int _B = 131072;
char buf[_B];
int _bl = 0;
inline void flush() {
    fwrite(buf, 1, _bl, stdout);
    _bl = 0;
}
__inline void _putchar(char c) {
    if(_bl == _B) flush();
    buf[_bl++] = c;
}
inline void print(LL x, char c) {
    static char tmp[20];
    int l = 0;
    if(!x) tmp[l++] = '0';
    else {
        while(x) {
            tmp[l++] = x % 10 + '0';
            x /= 10;
        }
    }
    for(int i = l - 1; i >= 0; i--) _putchar(tmp[i]);
    _putchar(c);
}
struct P {
    D x, y;
};
const int N = 300033;
const int LOG = 20;
const int mod = 1e9 + 7;
const int inf = 1e9 + 7;
int n, m;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
int main() {
    int n;
    scanf("%d", &n);
    int sum = 0;
    vector<pair<int, int> > v;
    for(int i = 1; i < n; i++) {
        v.pb({i, i});
        sum += i;
    }
    if(n % 2 == 1) v.pb({(n / 2 + sum) % 2 == 1 ? n / 2 + 1 : n / 2, n});
    else v.pb({n / 2, n});
    sort(all(v));
    vector<pair<int, int> > ans;
    while(!v.empty()) {
        //for(int i = 0; i < (int)v.size(); i++) printf("[%d %d]", v[i].fi, v[i].se);
        //printf("\n");
        auto t1 = v.back();
        v.pop_back();
        for(int j = v.size() - 1; t1.fi && j >= 0; j--) {
            ans.pb({t1.se, v[j].se});
            v[j].fi--;
            t1.fi--;
        }

        sort(all(v));
    }
    printf("%d\n", ans.size());
    for(auto i : ans) printf("%d %d\n", i.fi, i.se);

}