题解 | Forsaken喜欢数论

import java.util.*;

public class Main{
    static final int N = 30000005;
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        long ans = 0;
        boolean[] isprime = new boolean[N];
        for(int i=1;i<=n;i++) isprime[i] = true;
        isprime[0] = isprime[1] = false;
        for(int i=2;i<=n;i++){
            if(!isprime[i]) continue;
            ans += i;
            for(int j=2*i;j<=n;j+=i){
                if(!isprime[j]) continue;
                isprime[j] = false;
                ans += i;
            }
        }
        System.out.print(ans);
    }
}