Sequence Growing Easy

AtCoder

IDagc024_c

Time2000ms

Memory256MB

Difficulty—

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: $N$ $A_1$ $:$ $A_N$ [samples]

Samples

Input #1

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 $(0,1,1,2)$.

Input #2

Output #2

\-1

Input #3

Output #3

API Response (JSON)

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    "name": "Sequence Growing Easy",
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      "content": "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",
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      "statement_type": "Markdown",
      "content": "There is a sequence $X$ of length $N$, where every element is initially $0$. Let $X_i$ denote the $i$\\-th element of $X$.\nYou are given a sequence $A$ of length $N$. The $i$\\-th element of $A$ is $A_i...",
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