import std;

void main() {
    int N;
    readf("%d\n", N);

    auto P = readln.chomp.split.to!(long[]);

    long res;

    long tmp = N * (N - 1) / 2 - P.maxIndex - 1;
    if (res < tmp) {
        res = tmp;
    }

    foreach (i; 0 .. N) {
        tmp = P[i] * (P[i] + 1) / 2 - 2 * (i + 1);
        if (res < tmp) {
            res = tmp;
        }
    }

    res.writeln;
}