First duplicate

Given an array a that contains only numbers in the range from 1 to a.length, find the only duplicate number. If there are no such elements, return -1.

Example

• For a = [2, 1, 3, 4, 3], the output should be

  one_duplicate(a) = 3.
  

• For a = [2, 4, 3, 5, 1], the output should be one_duplicate(a) = -1.

Input/Output

• [input] array.integer a

  Guaranteed constraints:
  1 ≤ a.length ≤ 105,
  1 ≤ a[i] ≤ a.length.
  

• [output] integer The duplicated element. If there are no such elements, return -1.