Solution: Missing Number in Sorted Array

class MissingNumber {
 public static int missingNumber(int[] arr, int size) {
  int missing = -1; // if no element is missing
  int last = arr[0]; // start off from the first element
  if(last != 1)
  {
      return 1;
  }
  for (int i = 1; i < size; i++) { // traverse through the whole array
   if (arr[i] - last > 1) {
    missing = last + 1;
    break;
   }
   last++;
  }
  return missing;
 }
 public static void main(String args[]) {
  int[] input1 = {1,2,4};
  System.out.println(missingNumber(input1, input1.length));
 }
}