N,M = map(int,input().split())
B = list(map(int,input().split()))
PB = [-1]*M
S = 0
cnt = 0
for i in reversed(range(M)):
    if i == M-1:
        PB[i] = N - B[i]
        S += PB[i]
        cnt += 1
    else:
        p_walk = PB[i+1] + (B[i+1] - B[i]) #ワープしない場合
        p_warp = (S+M)*(M-cnt)/(cnt*(M-cnt))
        #print(i,S,M-cnt,p_walk,p_warp)
        if p_walk < p_warp:
            PB[i] = p_walk
            S += PB[i]
            cnt += 1
        else:
            PB[i] = p_warp
#print(PB)
ans = PB[0] + B[0] - 1
print(ans)