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

if m==1:
    ans=n-1
else:
    tmpsum=n-b[-1]
    for i in range(m-2,-1,-1):
        warp=(tmpsum+m)/(m-i-1)
        normal=n-b[i]
        if warp<normal:
            tmp=warp
            break
        else:
            tmpsum+=n-b[i]
    ans=b[0]-1+warp
print(ans)