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

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  ll N, K;
  cin >> N >> K;

  vector<ll> H(N);
  for(auto &i : H) { cin >> i; }
  H.emplace_back(0);

  vector<ll> v;
  ll l = H[0];
  for(ll i = 0; i < N; i++) {
    if(H[i] > H[i + 1]) {
      v.emplace_back(H[i] - l);
      l = H[i + 1];
    }
  }

  ll ans = 0;
  for(auto &i : v) { ans += min(i, K); }

  cout << ans << "\n";
}