def Bisect_Int(ok,ng,is_ok):
    while abs(ok-ng)>1:
        mid=(ok+ng)//2
        if is_ok(mid):
            ok=mid
        else:
            ng=mid
    return ok

N,K=map(int,input().split())
A=list(map(int,input().split()))
D=[b-a for a,b in zip(A,A[1:])]
D.append(2*N-sum(D))
mi=min(D)
i=D.index(mi)
D=D[i:]+D[:i]
for i in range(K):
    D[i]//=2
C=[0]+D
for i in range(1,K+1):
    C[i]+=C[i-1]
def is_ok(d):
    for s in range(C[1]):
        cur=s
        for k in range(1,K):
            cur+=d
            if cur<C[k]:
                cur=C[k]
            elif C[k+1]<=cur:
                return False
        cur+=d
        if cur<=s+N:
            return True
    return False
ans=Bisect_Int(0,N+1,is_ok)*2
print(ans)