A zero-indexed array A consisting of N different integers is given. The array contains integers in the range [1..(N + 1)], which means that exactly one element is missing.

Your goal is to find that missing element.

Write a function:
```
def solution(A)
```
that, given a zero-indexed array A, returns the value of the missing element.

For example, given array A such that:

  A[0] = 2
  A[1] = 3
  A[2] = 1
  A[3] = 5
the function should return 4, as it is the missing element.

Assume that:

+ N is an integer within the range [0..100,000];
+ the elements of A are all distinct;
+ each element of array A is an integer within the range [1..(N + 1)].
Complexity:

+ expected worst-case time complexity is O(N);
+ expected worst-case space complexity is O(1), beyond input storage (not counting the storage required for input arguments).
+ Elements of input arrays can be modified.