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

const int MAXN = int(2e5);

int N, P;
int H[MAXN];

long long dpl[MAXN], dpr[MAXN];

signed main() {
  ios::sync_with_stdio(false);
  cin >> N >> P;
  for (int i = 0; i < N; ++i) {
    cin >> H[i];
  }
  memset(dpl, 0x3f, sizeof(dpl));
  memset(dpr, 0x3f, sizeof(dpr));
  dpl[0] = dpr[0] = 0;
  for (int i = 1; i < N; ++i) {
    if (1 < i) {
      dpl[i] = min(dpl[i - 1] + min(P, max(0, H[i - 1] - H[i])), dpr[i - 1] + P);
    } 
    dpr[i] = min(dpr[i - 1] + min(P, max(0, H[i] - H[i - 1])), dpl[i - 1] + P);
  }
  cout << min(dpl[N - 1], dpr[N - 1]) << endl;
  return 0;
}