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#include <cassert>
#include <iostream>
#include <set>
#include <vector>

using namespace std;
using ll = long long;
#define all(a) begin(a), end(a)

void solve() {
    ll n, k;
    cin >> n >> k;
    vector<ll> a(n);
    for (auto &i : a) {
        cin >> i;
    }

    set<pair<ll, ll>> s;
    for (int i = 0; i < n; i++) {
        s.insert({a[i], i});
    }

    ll res = 0;
    while (!s.empty()) {
        auto [_, p] = *s.begin();
        s.erase(s.begin());

        ll val = a[p], lim = val - k;
        for (int d : {-1, +1}) {
            int i = p + d;
            if (i < 0 || i >= n) {
                continue;
            }

            if (a[i] < lim) {
                res += lim - a[i];
                a[i] = lim;
                s.insert({a[i], i});
            }
        }
    }
    cout << res << "\n";
}

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

    int q = 1;
    // cin >> q;
    while (q--) {
        solve();
    }
}