C = int(input())

C *= 1000

left = 0
right = 10**18
while left+1 < right:
    mid = (left+right)//2
    if (mid**5-1)//10**12+mid < C:
        left = mid
    else:
        right = mid

ans = list(str(left))
while len(ans) < 4:
    ans = ["0"]+ans
ans.insert(1, ".")
print("".join(ans))