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#include <iostream>
#include <vector>
#include <unordered_map>
#include <climits>

using namespace std;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(nullptr);
    
    int n, k;
    cin >> n >> k;
    vector<int> path(n, 0);
    for (int i = 0; i < n; i++) 
        cin >> path[i];

    vector<int> minReqRight(n, 0), minReqLeft(n, 0);
    minReqLeft[0] = path[0];
    minReqRight[n - 1] = path[n - 1];
    for (int i = 1, j = n - 2; i < n && j >= 0; i++, j--) {
        minReqLeft[i]  = max(minReqLeft[i - 1] - k, path[i]);
        minReqRight[j] = max(minReqRight[j + 1] - k, path[j]);
    }

    int answer = 0;
    for (int i = 0; i < n; i++) {
        int minPossible = max(minReqLeft[i], minReqRight[i]);
        answer += minPossible - path[i];
    }

    cout << answer << "\n";

    return 0;
}