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#include<bits/stdc++.h>

using namespace std;

int main() {
	std::ios_base::sync_with_stdio(false);
	std::cin.tie(NULL);
	
	int n, k;
	cin >> n >> k;
	
	vector<int> a(n);
	
	for (int i = 0; i < n; i++) {
		int ai;
		cin >> ai;
		a[i] = ai;
	}
	
	int r = 0;
	
	for (int i = 1; i < n; i++) { 
		if (a[i-1] - k > a[i]) {
			int b = a[i-1] - k - a[i];
			a[i] = a[i-1] - k;
			r += b;
		}
	}
	
	for (int i = n-1; i >=0; i--) {
		if (a[i+1] - k > a[i]) {
			int b = a[i+1] - k - a[i];
			a[i] = a[i+1] - k;
			r += b;
		}
	}
	
	cout << r;
}