#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(int (i) = (s);(i) <= (e);(i)++)

int N;
i64 P;
vector<i64> H;
int main(){
  cin >> N >> P;
  H.resize(N);
  rep(i,0,N - 1) cin >> H[i];
  vector<i64> dp(N + 10,1LL << 60);
  int from = 0;
  dp[0] = 0;
  for(int i = 1;i < N;i++){
    dp[i] = dp[i - 1] + min(P,max(0LL,H[i] - H[i - 1]));
    if(H[i] - H[i - 1] >= 0){
    }
    else{
      dp[i] = min(dp[i] , dp[from] + P + (i == N - 1 ? 0LL : P));
      from = i;
    }
  }
  if(from != N - 1){
      dp[N - 1] = min(dp[N - 1] , dp[from] + P + (N - 1 == N - 1 ? 0LL : P));
  }
  cout << dp[N - 1] << endl;
}