1 #include <iostream> 2 #include <cstdio> 3 #include <cstring> 4 using namespace std; 5  6 bool prime[1100000] = {0}; 7  8 void getlist(int maxint) { 9     prime[1] = 1;10     for(int i = 2 ; i <= maxint/2 ; ++i) {11         if(!prime[i])12             for(int j = i<<1 ; j <= maxint ; j += i)13                 prime[j] = 1;14     }15 }16 17 int main() {18     ios::sync_with_stdio(false);19     int n;20     cin >> n;21     getlist(n);22     int total = 0;23     for(int i = 3 ; i <= n ; ++i) {24         if(!prime[i] && !prime[i-2])25             ++total;26         if(!prime[i] && !prime[i-1])27             ++total;28     }29     cout << total << endl;    30     return 0;31 }