func_code_string: #include <bits/stdc++.h>   using namespace std;   long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }   inline void R(int &xx) {     xx = 0;     char ch = getchar();     while (ch <  0  || ch >  9 ) ch = getchar();     while (ch >=  0  && ch <=  9 )       xx = xx + xx + (xx << 3) + ch - 48, ch = getchar();   }   inline void R(long long &xx) {     xx = 0;     char ch = getchar();     while (ch <  0  || ch >  9 ) ch = getchar();     while (ch >=  0  && ch <=  9 )       xx = xx + xx + (xx << 3) + ch - 48, ch = getchar();   }   int n, prime[100011], tot;   bool f[100011], u[100011];   vector<pair<int, int> > ans;   vector<int> tmp;   inline void init() {     for (int i = 2; i <= n; i++) {       if (!f[i]) prime[++tot] = i;       for (int j = 1; j <= tot && i * prime[j] < 100011; j++) {         f[i * prime[j]] = 1;         if (!(i % prime[j])) break;       }     }   }   int main() {     R(n);     init();     for (int i = tot; i >= 1; i--) {       int pr = prime[i];       tmp.clear();       for (int j = pr; j <= n; j += pr)         if (!u[j]) tmp.push_back(j);       if (tmp.size() & 1) swap(tmp[1], tmp[tmp.size() - 1]);       for (int t = 0; t < ((int)tmp.size() - 1); t += 2) {         ans.push_back(make_pair(tmp[t], tmp[t + 1]));         u[tmp[t]] = u[tmp[t + 1]] = 1;       }     }     printf( %d n , ans.size());     for (int i = 0; i < ans.size(); i++)       printf( %d %d n , ans[i].first, ans[i].second);     return 0;   }   