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

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    ll n, k;
    cin >> n >> k;
    vector <ll> l(n);
    cin >> l[0];
    ll val = 0;
    for (ll i = 1; i < n; i++) {
        cin >> l[i];
        ll j = i - 1;
        while (l[j + 1] - l[j] > k) {
            val += l[j + 1] - l[j] - k;
            l[j] = l[j + 1]-k;
            j--;
            if (j < 0) {
                break;
            }
        }
        if (l[i - 1] - l[i] > k) {
            val += l[i - 1] - k - l[i];
            l[i] = l[i - 1] - k;
        }
    }
    cout << val << '\n';
}