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

int main() {

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

    vector<long long> v(n);
    vector<long long> original(n);

    for (int i = 0; i < n; i++) {
        cin >> v[i];
        original[i] = v[i];
    }


    for (int i = 0; i < n - 1; i++) {
        if (v[i] - k > v[i+1]) {
            v[i+1] = v[i] - k;
        }
    }

    
    for (int i = n - 2; i >= 0; i--) {
        if (v[i+1] - k > v[i]) {
            v[i] = v[i+1] - k;
        }
    }

    long long ciezarowki = 0;
    for (int i = 0; i < n; i++) {
        ciezarowki += (v[i] - original[i]);
    }

    cout << ciezarowki << endl;

    return 0;
}