#include <iostream>

using namespace std;

int main()
{
    long long int n, k;
    cin >> n >> k;
    long long int hany = 0;
    for(long long int i=0; i<n; i++){
        int h;
        cin >> h;
        hany+=h;
    }
    if(hany>=k){
        cout << (hany-k+1)*(hany-k+2)/2;
    }
    else{
        cout << 0;
    }
}