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#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n;
    long long k;
    cin >> n >> k;

    vector<long long> b(n);
    for (int i = 0; i < n; i++) cin >> b[i];

    vector<long long> a(b);

    for (int i = 1; i < n; i++)
        b[i] = max(b[i], b[i-1] - k);

    for (int i = n-2; i >= 0; i--)
        b[i] = max(b[i], b[i+1] - k);

    long long ans = 0;
    for (int i = 0; i < n; i++)
        ans += b[i] - a[i];

    cout << ans << "\n";
    return 0;
}