#include <cstdio>

using namespace std;

inline int readInt() {
    int x = 0;
    int c = getchar_unlocked();
    while (c != EOF && (c < '0' || c > '9')) c = getchar_unlocked();
    while (c != EOF && (c >= '0' && c <= '9')) {
        x = x * 10 + (c - '0');
        c = getchar_unlocked();
    }
    return x;
}

int main() {
    int n = readInt();
    int k = readInt();

    int o = 0;
    for (int i = 0; i < n; ++i) o+=readInt();

    if (o >= k) {
        unsigned long long m = (unsigned long long)o - k + 1;
        
        printf("%llu\n", m * (m + 1) / 2);
    } else {
        printf("0\n");
    }

    return 0;
}