#include<bits/stdc++.h>
#define int long long
using namespace std;
#define fi first
#define sc second
#define pii pair<int,int>
#define pdd pair<double,double>
#define pb push_back
#define umap unordered_map
#define mset multiset
#define pq priority_queue
#define ull unsigned long long
#define i128 __int128
#define ld long double
#define fixs fixed<<setprecision
#define FileIn(x) freopen(x".in","r",stdin)
#define FileOut(x) freopen(x".out","w",stdout)
#define FileIO(x) FileIn(x),FileOut(x);
const int maxn=1e6+10;
int n,p,a[maxn],f[maxn][2];
void solve(){
    cin>>n>>p,f[1][1]=1e18;
    for(int i=1;i<=n;i++) cin>>a[i];
    for(int i=2;i<=n;i++) f[i][0]=min(f[i-1][1]+p,f[i-1][0]+min(p,max(a[i]-a[i-1],0ll))),f[i][1]=min(f[i-1][0]+p,f[i-1][1]+min(p,max(a[i-1]-a[i],0ll)));
    cout<<min(f[n][0],f[n][1])<<endl;
}
signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0),cout.tie(0);
    int t=1;
    // cin>>t;
    while(t--) solve();
    return 0;
}
/*
Samples
input:

output:

THINGS TODO:
??freopen???????
????
????????????
*/