# First Element Equals its Index

Let's check another problem with a sorted input array. Difficulty Level: Easy

## We'll cover the following

## Problem statement

Suppose we are given a **sorted array** of distinct integers, `nums`

, which are sorted in ascending order. We will return the **smallest index** `i`

, which satisfies `nums[i] == i`

. If no such `i`

exists, we will return `-1`

.

**Constraints:**

- $1 \leq nums.length \leq 10^3$
- $-10^3 \leq nums[i] \leq 10^3$

**Example 1: General case**

```
Input: [-10,-5,0,3,7]
Output: 3
```

**Explanation:** For the given array, `nums[0] = -10`

, `nums[1] = -5`

, `nums[2] = 0`

, `nums[3] = 3`

, the output is `3`

.

**Example 2: The answer is at the beginning of the array**

```
Input: [0,2,5,8,17]
Output: 0
```

**Explanation:** `nums[0] = 0`

. Thus the output is `0`

.

**Example 3: The answer is missing**

```
Input: [-10,-5,3,4,7,9]
Output: -1
```

**Explanation:** There is no such `i`

that `nums[i] = i`

. Thus the output is `-1`

.

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