C=int(input())
left,right = 0, 10**17
while right-left > 1:
    mid = (right+left)//2
    if mid**5//(10**16) + mid <= C*10**4:
        left = mid
    else:
        right = mid
print(str(left//10**4)+"."+str(left//10**3%10)+str(left//10**2%10)+str(left//10%10))