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#include <bits/stdc++.h>
using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n;
    long long k;
    cin >> n >> k;

    vector<long long> a(n);
    for (int i = 0; i < n; i++) cin >> a[i];

    vector<long long> b(a);

    for (int i = 1; i < n; i++) {
        b[i] = max(b[i], b[i-1] - k);
    }

    for (int i = n-2; i >= 0; i--) {
        b[i] = max(b[i], b[i+1] - k);
    }

    long long ans = 0;
    for (int i = 0; i < n; i++) {
        ans += b[i] - a[i];
    }

    cout << ans << endl;

    return 0;
}