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

int main() {
    int n, k;
    cin >> n >> k;

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

    for (int i = 1; i < n; i++) {
        if (a[i] < a[i-1] - k) {
            a[i] = a[i-1] - k;
        }
    }

    for (int i = n-2; i >= 0; i--) {
        if (a[i] < a[i+1] - k) {
            a[i] = a[i+1] - k;
        }
    }

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

    cout << ans << endl;
}