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//Solution by Mikołaj Kołek

#include "bits/stdc++.h"
#define intin *istream_iterator<int>(cin)

using namespace std;

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	
	int n = intin, k = intin;
	vector<int> levels(n);
	copy_n(istream_iterator<int>(cin), n, levels.begin());
	
	int score = 0;
	for(int i = 1; i < n; i++) {
		if(levels[i] < (levels[i - 1] - k)) {
			int difference = (levels[i - 1] - k) - levels[i];
			levels[i] += difference;
			score += difference;
		}
		
		int pos = i - 1;
		while(pos != -1 and levels[pos] < (levels[pos + 1] - k)) {
			int difference = (levels[pos + 1] - k) - levels[pos];
			levels[pos] += difference;
			score += difference;
			pos--;
		}
	}
	
	cout << score;
	
	return 0;
}