#yuki_595

n,p=map(int,raw_input().split())
h=map(int,raw_input().split())
l=0
r=float('inf')
for i in xrange(1,n):
 l=min((min(l,r)+p),l+max(h[i]-h[i-1],0))
 r=min((min(l,r)+p),r+max(h[i-1]-h[i],0))
print min(l,r)