#include <bits/stdc++.h>

using namespace std;

int main() {

    cin.tie(0);
    ios::sync_with_stdio(0);

    long long n, k;
    long long s;
    cin >> n >> k;
    long long retek = 0, utvonalak = 0;

    while(n--)
    {
        cin >> s;
        retek += s;
    }
    //cout << retek << endl;
    
    if(retek < k)
    {
        cout << "0\n";
        exit(0);
    }

    for(long long i = 1; i <= retek-k+1; i++)
    {
        //cout << "asd: " << retek-i-k+2 << endl;
        utvonalak += retek-i-k+2;
    }
    cout << utvonalak << "\n";
    return 0;
}