sznoi b003

 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 }