Hi,
The problem description is in the comments above the solution below.

The problem belongs to Codility, you can try out it yourself on their platform.
 

/**
 * An 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:
 * class Solution { public int solution(int[] A); }
 * that, given an 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.
 */
public class FindMissing {
    public int solution(int[] A) {
        long sum = 0;
        long n = A.length;
        for (int i = 0; i < n; i++) {
            sum += A[i];
        }
        n = n + 1;
        return (int) ((n * (n + 1)) / 2 - sum);
    }
}