﻿ AGC024 C - Sequence Growing Easy - Atcoder

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Score : $700$ points

### Problem Statement

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.

• Choose an integer $i$ such that $1\leq i\leq N-1$. Replace the value of $X_{i+1}$ with the value of $X_i$ plus $1$.

### Constraints

• $1 \leq N \leq 2 \times 10^5$
• $0 \leq A_i \leq 10^9(1\leq i\leq N)$
• All values in input are integers.

### Input

Input is given from Standard Input in the following format:

$N$
$A_1$
$:$
$A_N$


### Output

If we can make $X$ equal to $A$ by repeating the operation, print the minimum number of operations required. If we cannot, print $-1$.

### Sample Input 1

4
0
1
1
2


### Sample Output 1

3


We can make $X$ equal to $A$ as follows:

• Choose $i=2$. $X$ becomes $(0,0,1,0)$.
• Choose $i=1$. $X$ becomes $(0,1,1,0)$.
• Choose $i=3$. $X$ becomes $(0,1,1,2)$.

### Sample Input 2

3
1
2
1


### Sample Output 2

-1


### Sample Input 3

9
0
1
1
0
1
2
2
1
2


### Sample Output 3

8