虽然说蒟蒻AC了，但是还想找大佬求一下正解

代码：

#include <iostream>
#include <vector>
const int MAXN = 10e9+10;
using namespace std;

vector<bool> present(MAXN, false);

int main() {
    int N, K;
    cin >> N >> K;

    for (int i = 0; i < N; i++) {
        int num;
        cin >> num;
        present[num] = true;
    }

    long long sum = 0;
    for (int i = 1; i <= K; i++) {
        if (!present[i]) {
            sum += i;
        }
    }

    cout << sum << endl;

    return 0;
}