#pragma GCC optimize ("O3")

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <queue>
#include <set>
#include <algorithm>
#include <numeric>
using namespace std;

using QWORD  = uint64_t;
using SQWORD = int64_t;
using DWORD  = uint32_t;
using SDWORD = int32_t;
using WORD   = uint16_t;
using SWORD  = int16_t;
using BYTE   = uint8_t;
using SBYTE  = int8_t;
using DOUBLE = double;
using FLOAT  = float;

#define MIN_SDWORD (-2147483648)
#define MAX_SDWORD (2147483647)
#define MIN_SBYTE (-128)
#define MAX_SBYTE (127)

#define MIN_SQWORD (0x8000000000000000)
#define MAX_SQWORD (0x7FFFFFFFFFFFFFFF)

#define MAX_QWORD  (0xFFFFFFFFFFFFFFFF)
#define MAX_DWORD  (0xFFFFFFFF)
#define MAX_WORD   (0xFFFF)
#define MAX_BYTE   (0xFF)


#define ArrayLength(a)  (sizeof(a) / sizeof(a[0]))

static inline QWORD MAX(QWORD a, QWORD b) { return a > b ? a : b; }
static inline DWORD MAX(DWORD a, DWORD b) { return a > b ? a : b; }
static inline SDWORD MAX(SDWORD a, SDWORD b) { return a > b ? a : b; }
static inline QWORD MIN(QWORD a, QWORD b) { return a < b ? a : b; }
static inline DWORD MIN(DWORD a, DWORD b) { return a < b ? a : b; }
static inline SDWORD MIN(SDWORD a, SDWORD b) { return a < b ? a : b; }

#define BYTE_BITS   (8)
#define WORD_BITS   (16)
#define DWORD_BITS  (32)
#define QWORD_BITS  (64)

using M_BOOL = bool;
#define M_TRUE (true)
#define M_FALSE (false)
#define DIVISOR (1000000007)

static inline void inputString(char *pcStr)
{
    char *pcCur = pcStr;
    for (;;) {
        char c = getchar();
        if (('\n' == c) || (EOF == c)) {
            break;
        }
        *pcCur = c;
        pcCur++;
    }
    *pcCur = '\0';
}


static inline SQWORD inputSQWORD(void)
{
    SQWORD sqNumber = 0;
    SQWORD sqMultiplier = 1;
    M_BOOL bRead = M_FALSE;
    for (;;) {
        char c = getchar();
        if (!bRead) {
            if ('-' == c) {
                sqMultiplier = -1;
            }
        }
        if (('0' <= c) && (c <= '9')) {
            sqNumber *= 10LL;
            sqNumber += (SQWORD)(c - '0');
            bRead = M_TRUE;
        } else {
            if (bRead) {
                return sqNumber * sqMultiplier;
            }
        }
    }
}


static inline SDWORD inputSDWORD(void)
{
    SDWORD lNumber = 0;
    SDWORD lMultiplier = 1;
    M_BOOL bRead = M_FALSE;
    for (;;) {
        char c = getchar();
        if (!bRead) {
            if ('-' == c) {
                lMultiplier = -1;
            }
        }
        if (('0' <= c) && (c <= '9')) {
            lNumber *= 10;
            lNumber += (c - '0');
            bRead = M_TRUE;
        } else {
            if (bRead) {
                return lNumber * lMultiplier;
            }
        }
    }
}

static inline DOUBLE inputFP(void)
{
    DOUBLE dInt = 0.0;
    DOUBLE dFrac = 0.0;
    DOUBLE dMultiplier = 1.0;
    DWORD dwFpCnt = 0;
    DOUBLE *pdCur = &dInt;
    M_BOOL bRead = M_FALSE;
    for (;;) {
        char c = getchar();
        if (!bRead) {
            if ('-' == c) {
                dMultiplier = -1;
            }
        }
        if ('.' == c) {
            pdCur = &dFrac;
        } else if (('0' <= c) && (c <= '9')) {
            (*pdCur) *= 10;
            (*pdCur) += (DOUBLE)(c - '0');
            bRead = M_TRUE;
            if (pdCur == &dFrac) {
                dwFpCnt++;
            }
        } else {
            if (bRead) {
                return dMultiplier * (dInt + dFrac / (pow((DOUBLE)10.0 , (DOUBLE)dwFpCnt)));
            }
        }
    }
}


#define MAX_STONES  (200000)
#define MAX_COLOR   (200000)
static SQWORD s_asqLastIdx[MAX_STONES + 1];
static SQWORD s_asqCumSum[MAX_STONES + 1];
#define ANS_MOD         (1000000007)


int main()
{
    SDWORD lInput_N = inputSDWORD();

    SDWORD lSum = lInput_N * (lInput_N + 1) / 2;
    SDWORD lDiv = 0;
    for (lDiv = 2; lDiv <= lSum / 2; lDiv++) {
        if (lSum % lDiv== 0) {
            break;
        }
    }

    vector<SDWORD> vVertice;
    for (SDWORD lNum = lInput_N; 0 < lNum; --lNum) {
        vVertice.emplace_back(lNum);
    }

    vector<SDWORD> vVertex_list[100];
    for (DWORD dwIdx = 0; dwIdx < 100; dwIdx++) {
        if (0 == vVertice.size()) {
            break;
        }
//        printf("------\n");
        SDWORD lRest = lSum / lDiv;
        vector<SDWORD> vSubIndice;
        for (auto it = vVertice.begin(); it != vVertice.end(); ++it) {
            if (*it <= lRest) {
                vSubIndice.emplace_back(it - vVertice.begin());
                vVertex_list[dwIdx].emplace_back(*it);
                lRest -= *it;
//                printf("--> %d\n", *it);
            }
        }
        reverse(vSubIndice.begin(), vSubIndice.end());
        for (auto it: vSubIndice) {
            vVertice.erase(vVertice.begin() + it);
        }
    }

    DWORD dwNum = 0;
    for (DWORD dwIdxI = 0; dwIdxI < lDiv; dwIdxI++) {
        for (DWORD dwIdxJ = 0; dwIdxJ < dwIdxI; dwIdxJ++) {
            for (auto it_f: vVertex_list[dwIdxI]) {
                for (auto it_j: vVertex_list[dwIdxJ]) {
                    dwNum++;
                }
            }
        }
    }

    printf("%d\n", dwNum);
    for (DWORD dwIdxI = 0; dwIdxI < lDiv; dwIdxI++) {
        for (DWORD dwIdxJ = 0; dwIdxJ < dwIdxI; dwIdxJ++) {
            for (auto it_f: vVertex_list[dwIdxI]) {
                for (auto it_j: vVertex_list[dwIdxJ]) {
                    printf("%d %d\n", it_f, it_j);
                }
            }
        }
    }

 
    return 0;
}

Measured memory_bytes: 256