import sys,random,bisect
from collections import deque,defaultdict
from heapq import heapify,heappop,heappush
from itertools import permutations
from math import gcd,log


N,M = map(int,input().split())
B = list(map(int,input().split()))

def cond(x):
    val = [0 for i in range(M)]
    val[-1] = min(1+x,N-B[-1])
    for i in range(M-2,-1,-1):
        val[i] = min(1+x,B[i+1]-B[i]+val[i+1])

    check = sum(val)/M
    return check>=x

ok = 0
ng = N
T = 100
while T:
    mid = (ok+ng)/2
    if cond(mid):
        ok = mid
    else:
        ng = mid
    T -= 1

x = ok
val = [0 for i in range(M)]
val[-1] = min(1+x,N-B[-1])
for i in range(M-2,-1,-1):
    val[i] = min(1+x,B[i+1]-B[i]+val[i+1])

print(val[0]+B[0]-1)