Score : 700 points
There is a sequence X of length N, where every element is initially 0. Let X_i denote the i-th element of X.
You are given a sequence A of length N. The i-th element of A is A_i. Determine if we can make X equal to A by repeating the operation below. If we can, find the minimum number of operations required.
Input is given from Standard Input in the following format:
N A_1 : A_N
If we can make X equal to A by repeating the operation, print the minimum number of operations required. If we cannot, print -1.
4 0 1 1 2
3
We can make X equal to A as follows:
3 1 2 1
-1
9 0 1 1 0 1 2 2 1 2
8