素数对猜想 (20)

 1 #include <iostream>
 2 using namespace std;
 3 
 4 bool isprime(int n){
 5     if (n <= 1) return false;
 6     for (int i = 2; i*i <= n; i++)
 7     if (n%i == 0) 
 8         return false;
 9     return true;
10 }
11 
12 int main(){
13     int n, sum = 0, lastprime = 2;
14     cin >> n;
15     for (int i = 1; i <= n; i++){
16         if (isprime(i)){
17             if (i - lastprime == 2) sum++;
18             lastprime = i;
19         }
20     }
21     cout << sum;
22     return 0;
23 }