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#include <bits/stdc++.h>
using namespace std;
int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	
	int n, k;
	cin >> n >> k;
	vector<int> a(n);
	long long sum = 0;
	for (int i = 0; i < n; i++) {
		cin >> a[i];
		sum += a[i];
	}
	for (int rep = 0; rep < 2; rep++) {
		for (int i = 1; i < n; i++) {
			a[i] = max(a[i], a[i-1] - k);
		}
		reverse(a.begin(), a.end());
	}
	for (int x : a) sum -= x;
	cout << -sum << endl;
}