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

int main(){
	ios_base::sync_with_stdio(false), cin.tie(nullptr);
	int N, K;
	cin >> N >> K;
	vector<int> A(N);
	for(int i = 0; i < N; i++){
		cin >> A[i];
	}
	vector<int> bound = A;
	for(int i = 1; i < N; i++){
		bound[i] = max(bound[i], bound[i-1] - K);
	}
	for(int i = N-1; i > 0; i--){
		bound[i-1] = max(bound[i-1], bound[i] - K);
	}
	int ans = 0;
	for(int i = 0; i < N; i++) ans += bound[i] - A[i];
	cout << ans << '\n';
}