C1 - Triangular Lamps Easy

AtCoder

IDwtf19_c1

Time2000ms

Memory256MB

Difficulty—

There is an infinitely large triangular grid, as shown below. Each point with integer coordinates contains a lamp. ![image](https://img.atcoder.jp/wtf19/f617c94527a62ed72fe7db12b6d1f6b0.png) Initially, only the lamp at $(X, 0)$ was on, and all other lamps were off. Then, Snuke performed the following operation zero or more times: * Choose two integers $x$ and $y$. Toggle (on to off, off to on) the following three lamps: $(x, y), (x, y+1), (x+1, y)$. After the operations, $N$ lamps $(x_1, y_1), \cdots, (x_N, y_N)$ are on, and all other lamps are off. Find $X$. ## Constraints * $1 \leq N \leq 10^5$ * $-10^{17} \leq x_i, y_i \leq 10^{17}$ * $(x_i, y_i)$ are pairwise distinct. * The input is consistent with the statement, and you can uniquely determine $X$. ## Input Input is given from Standard Input in the following format: $N$ $x_1$ $y_1$ $:$ $x_N$ $y_N$ [samples]

Samples

Input #1

Output #1

\-1

The following picture shows one possible sequence of operations:
![image](https://img.atcoder.jp/wtf19/cff6dc4d81e995e9300ccbaca5bf85de.png)

API Response (JSON)

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    "name": "C1 - Triangular Lamps Easy",
    "description": {
      "content": "There is an infinitely large triangular grid, as shown below. Each point with integer coordinates contains a lamp. ![image](https://img.atcoder.jp/wtf19/f617c94527a62ed72fe7db12b6d1f6b0.png) Initially",
      "description_type": "Markdown"
    },
    "platform": "AtCoder",
    "limit": {
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      "memory_limit": 262144
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    "difficulty": "None",
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    "sign": "wtf19_c1"
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  "statements": [
    {
      "statement_type": "Markdown",
      "content": "There is an infinitely large triangular grid, as shown below. Each point with integer coordinates contains a lamp.\n![image](https://img.atcoder.jp/wtf19/f617c94527a62ed72fe7db12b6d1f6b0.png)\nInitially...",
      "is_translate": false,
      "language": "English"
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Full JSON Raw Segments