#include <bits/stdc++.h>

using namespace std;

using ll = long long;

ll h[200005], a[200005];

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n;
    ll p;
    cin >> n >> p;
    for (int i = 0; i < n; i++) {
        cin >> h[i];
    }
    for (int i = n - 2; i >= 0; i--) {
        a[i] = a[i + 1] + min(max(h[i] - h[i + 1], 0LL), p);
    }
    ll cur = 0, ans = 1000000000000000LL;
    for (int i = 0; i < n - 1; i++) {
        ans = min(ans, cur + a[i]);
        cur += max(h[i + 1] - h[i], 0LL);
    }
    ans = min(ans, cur);
    cout << ans << endl;
    return 0;
}