#include <bits/stdc++.h>

using namespace std;

using ll = long long;

ll h[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];
    }
    ll dp = 1000000000000000LL, dp2 = 0;
    for (int i = 1; i < n; i++) {
        ll tmp = min(dp + min(max(h[i - 1] - h[i], 0LL), p), dp2 + p);
        ll tmp2 = min(dp + p, dp2 + min(max(h[i] - h[i - 1], 0LL), p));
        dp = tmp;
        dp2 = tmp2;
    }
    cout << min(dp, dp2) << endl;
    return 0;
}