import sys
input = sys.stdin.readline

N, M = map(int, input().split())
B = list(map(int, input().split()))
ans = B[0]-1+N-B[-1]+M
acc = 0

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

print(ans)