B'You are given a sequence a_1, a_2, ... , a_n consisting of n integers. You can choose any non-negative integer D (i.e. D ge 0 ), and for each a_i you can: It is possible that after an operation the value a_i becomes negative. Your goal is to choose such minimum non-negative integer D and perform changes in such a way, that all a_i are equal (i.e. a_1=a_2= ... =a_n ). Print the required D or, if it is impossible to choose such value D , print -1. For example, for array [2, 8] the value D=3 is minimum possible because you can obtain the array [5, 5] if you will add D to 2 and subtract D from 8 . And for array [1, 4, 7, 7] the value D=3 is also minimum possible. You can add it to 1 and subtract it from 7 and obtain the array [4, 4, 4, 4] . The first line of the input contains one integer n ( 1 <= n <= 100 ) -- the number of elements in a . The second line of the input contains n integers a_1, a_2, ... , a_n ( 1 <= a_i <= 100 ) -- the sequence a . Print one integer -- the minimum non-negative integer value D such that if you add this value to some a_i , subtract this value from some a_i and leave some a_i without changes, all obtained values become equal. If it is impossible to choose such value D , print -1. '...