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#include <bits/stdc++.h>
using namespace std;
using i64 = long long;

int main() {
    int n, k;
    cin >> n >> k;

    vector<int> a(n);
    for (int &x: a) cin >> x;

    i64 res = 0;
    for (int i = 0; i+1 < n; i++) {
        if (a[i]-a[i+1] > k) {
            int d = a[i]-a[i+1]-k;
            res += d, a[i+1] += d;
        }
    }
    for (int i = n-2; i >= 0; i--) {
        if (a[i+1]-a[i] > k) {
            int d = a[i+1]-a[i]-k;
            res += d, a[i] += d;
        }
    }
    cout << res << '\n';
}