R=range
J=lambda:map(int,input().split())
N,K=J()
A=list(J())
A+=[a+N*2for a in A]
s=0
d=9**9
for i in R(K):
	if d>A[i+1]-A[i]:s,d=i,A[i+1]-A[i]
l,r=1,N+1
while l<r-1:
	m=(l+r)>>1;d=m*2
	for j in R(A[s]+1,A[s+1],2):
		t=j
		for i in R(1,K):
			if A[s+i+1]<=t+d:break
			t=max(A[s+i]+1,t+d)
		else:
			if t+d<=j+N*2:l=m;break
	else:r=m
print(l*2)