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

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
//-------------------------------------------------------

int N;
ll P,H[202020];
int nex[202020];

void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N>>P;
	FOR(i,N) cin>>H[i];
	
	nex[N-1]=nex[N]=N;
	for(i=N-1;i>=0;i--) {
		if(H[i]<=H[i+1]) nex[i]=nex[i+1];
		else nex[i]=i;
	}
	
	ll ret=0;
	int cur=0;
	while(cur<N-1) {
		if(H[cur]>=H[cur+1]) {
			cur++;
			continue;
		}
		if(nex[cur]==N-1) {
			ret+=min(H[N-1]-H[cur],P);
			break;
		}
		else if(nex[cur]==cur+1) {
			ret+=min(H[cur+1]-H[cur],P);
			cur++;
		}
		else {
			ret+=min(H[nex[cur]]-H[cur],2*P);
			cur=nex[cur];
		}
	}
	cout<<ret<<endl;
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}