python求平方根

x = int(input())
low, high, ans = 0, x, -1
while low <= high:
mid = (low + high) // 2
if mid * mid <= x:
ans = mid
low = mid + 1
else:
high = mid - 1
print(ans)

請問第四行low+high，low既然是0，為什麼不可以省略，我試了下，省略後電腦死循環。什麼原理？