素数对猜想 (20)

#include <iostream>
using namespace std;

bool isprime(int n){
    if (n <= 1) return false;
    for (int i = 2; i*i <= n; i++)
    if (n%i == 0) 
        return false;
    return true;
}

int main(){
    int n, sum = 0, lastprime = 2;
    cin >> n;
    for (int i = 1; i <= n; i++){
        if (isprime(i)){
            if (i - lastprime == 2) sum++;
            lastprime = i;
        }
    }
    cout << sum;
    return 0;
}