Given a non-negative integer `x`, return the **square root** of `x` rounded down to the nearest integer. The returned integer should be non-negative as well.
You must not use any built-in exponent function or operator.
**Example 1:**
```
Input: x = 4
Output: 2
```
**Example 2:**
```
Input: x = 8
Output: 2
Explanation: The square root of 8 is 2.82...and rounded down to the nearest integer it is 2.
```