n, m = map(int, input().split())
B = list(map(int, input().split()))

A = [0]*m
for i, b in enumerate(B):
    A[i] = n-b
C = [0]+A
from itertools import accumulate
C = list(accumulate(C))
ans = float('inf')
for i in range(m):
    y = m*(C[m]-C[i]+i)
    if (m-i)*A[i] <= m*(C[m]-C[i]+i):
        x = y/(m-i)
        ans = min(ans, B[0]-1+min(x/m+1, n-B[0]))
print(ans)