#include <algorithm>
int solution(vector<int> &A) {
    int N = A.size();
    unsigned long long sum = 0;
    for(unsigned int i = 0; i < A.size(); i++) {
        if(A[i] != N+1) {
            sum += A[i];
        }
    }

    unsigned long long sumShouldEqual = N * (N+1) / 2;
    int result = sumShouldEqual - sum;
    return result;
}