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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n, p;
    cin >> n >> p;
    vector<int> a(n);
    for(auto &&v : a) cin >> v;
    vector<ll> dp(n, 1ll << 60);
    vector<ll> dp2(n, 1ll << 60);
    dp[0] = 0;
    for(int i = 1; i < n; i++){
        dp[i] = min(dp[i], dp[i - 1] + min(p, max(0, a[i] - a[i - 1])));
        dp[i] = min(dp[i], dp2[i - 1] + p);
        dp2[i] = min(dp2[i], dp[i - 1] + p);
        dp2[i] = min(dp2[i], dp2[i - 1] + min(p, max(0, a[i - 1] - a[i])));
    }
    cout << min(dp[n - 1], dp2[n - 1]) << '\n';
}