#yuki_595

INF=float('inf')
dp=[[INF for i in xrange(2)] for j in xrange(200000)]
n,p=map(int,raw_input().split())
h=map(int,raw_input().split())
dp[0][0]=0
for i in xrange(n-1):
 dp[i+1][0]=min(dp[i+1][0],dp[i][0]+min(p,max(h[i+1]-h[i],0)))
 dp[i+1][1]=min((dp[i+1][1],dp[i][0]+p))
 dp[i+1][1]=min(dp[i+1][1],dp[i][1]+min(p,max(h[i]-h[i+1],0)))
 dp[i+1][0]=min(dp[i+1][0],dp[i][1]+p)
print min(dp[n-1][0],dp[n-1][1])