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

long long n, k;
vector<long long> a;
long long wyn = 0;
int main() {
	cin.tie(0)->sync_with_stdio(0);
	cin >> n >> k;
	for (int i = 0; i < n; i++) {
		int c;
		cin >> c;
		a.push_back(c);
	}
	
	bool bul = 1;
	while (bul) {
		bul = 0;
		for (int i = 0; i < n - 1; i++) {
			if (a[i] - a[i + 1] > k) {
				wyn += ((a[i] - k) - a[i + 1]);
				a[i + 1] = a[i] - k;
				bul = 1;
			}
		}

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

	cout << wyn << '\n';
}