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

int start_heights[1000010];
bool processed[1000010];

int main() {
	std::ios_base::sync_with_stdio(false);

	int n, k;

	priority_queue<pair<int, int>> required_heights;
	cin >> n >> k;

	int height;
	for (int i = 0; i < n; i++) {
		cin >> height;
		required_heights.push({height, i});
		processed[i] = false;
		start_heights[i] = height;
	}

	int result = 0;

	while(!required_heights.empty()) {
		auto aux = required_heights.top();
		required_heights.pop();
		int height = aux.first;
		int pos = aux.second;

		if (processed[pos]) {
			continue;
		}

		processed[pos] = true;
		result += height - start_heights[pos]; // Guaranteed to be non negative

		if (pos > 0) {
			required_heights.push({height - k, pos - 1});
		}
		if (pos < n - 1) {
			required_heights.push({height - k, pos + 1});
		}
	}
	cout << result << "\n";
}