text: #include <iostream>
  #include <vector>
  #include <algorithm>
  #include<numeric>
  
  
  using namespace std;
  
  int solution(vector<int> &A)
  {
      return (((A.size()+1)*(A.size()+2)/2) - (accumulate(A.begin(),A.end(),0)));
  }
  
  int main()
  {
      vector<int> A = {2,3,1,5};
      solution(A);
  }
  