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

int main() {
    ios::sync_with_stdio(false); cin.tie(nullptr);
    int n;
    long long k;
    cin >> n >> k;

    vector<long long> a(n + 1), L(n + 1), R(n + 1);

    for (int i = 1; i <= n; i++) cin >> a[i];
    

    L[1] = a[1];
    for (int i = 2; i <= n; i++) 
    {
        L[i] = max(a[i], L[i - 1] - k);
    }

    R[n] = a[n];

    for (int i = n - 1; i >= 1; i--) 
    {
        R[i] = max(a[i], R[i + 1] - k);

    }

    long long ans = 0;
    for (int i = 1; i <= n; i++) 
    {
        long long b = max(L[i], R[i]);
        ans += b - a[i];
    }

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