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

int main() {
    int n, k, tmp;
    cin >> n >> k;
    vector <int> a(n);
    int res = 0;
    for (int i = 0; i < n; ++i) { cin >> a[i]; }
    for (int i = 0; i < n; ++i) {
        if (i != n - 1) {
            if (a[i] - a[i + 1] > k) {
                tmp = a[i + 1];
                a[i + 1] = a[i] - k;
                res += a[i + 1] - tmp;
            } else if (a[i + 1] - a[i] > k) {
                int j = i;
                while (a[j + 1] - a[j] > k && j >= 0) {
                    tmp = a[j];
                    a[j] = a[j + 1] - k;
                    res += a[j] - tmp;
                    --j;
                } 
            }
        }
    }
    cout << res << '\n';
}