import sys

sys.setrecursionlimit(10**7)
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))
def LI2(): return list(map(int,sys.stdin.readline().rstrip()))
def S(): return sys.stdin.readline().rstrip()
def LS(): return list(sys.stdin.readline().rstrip().split())
def LS2(): return list(sys.stdin.readline().rstrip())


N,M = MI()
B = LI()

if M == 1:
    print(N-1)
    exit()

a = B[0]-1
B = [b-a for b in B]
N -= a
a += N-B[-1]

b = 0
ans = a+B[-1]-1
for i in range(M-1,-1,-1):
    b += B[-1]-B[i]
    ans = min(ans,a+M/(M-i)+b/(M-i))

print(ans)