#include <bits/stdc++.h>
using namespace std;
class Solution{
  public:
    int MissingNumber(vector<int>& array, int n) {
        // Your code goes here
        int sumOfNaturalNumbers = n * (n + 1)/2;
        int sumOfNumbersInArray = accumulate(array.begin(), array.end(), 0);
        return sumOfNaturalNumbers - sumOfNumbersInArray;
    }
};

int main() {
    vector<int> array{1,2,3,5};
    Solution obj;
    cout << obj.MissingNumber(array, 5) << "\n";
    return 0;
}