def solve():
N,L=map(int,input().split())
a=[0]+[L-int(input())for _ in[0]*N]
l=[]
cumsum,dir=0,0
for n,prev in zip(a[1:],a):
if dir==0 and n>prev or dir==1 and n<prev:
cumsum+=n
else:
l.append(cumsum)
cumsum=n+prev
dir^=1
else:
l.append(cumsum)
print(max(l))
if __name__=="__main__":
solve()
()
