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()
B.reverse()


def f(a):
    cur = N
    ex = 0
    s = 0
    for i in range(M):
        b = B[i]
        ex += cur-(b+1)
        m = min(a+1,ex+1)
        ex = m
        s += m
        cur = b
    ex += cur-1

    if s >= a*M:
        return True,ex
    return False,ex


ok = 0
ng = 10**10
while ng-ok >= 10**-5:
    mid = (ok+ng)/2
    b,e = f(mid)
    if b:
        ok = mid
    else:
        ng = mid

print(f(ok)[1])