{"problem":{"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":{"time_limit":2000,"memory_limit":262144},"difficulty":"None","is_remote":true,"is_sync":true,"sync_url":null,"sign":"wtf19_c1"},"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, only the lamp at $(X, 0)$ was on, and all other lamps were off. Then, Snuke performed the following operation zero or more times:\n\n*   Choose two integers $x$ and $y$. Toggle (on to off, off to on) the following three lamps: $(x, y), (x, y+1), (x+1, y)$.\n\nAfter the operations, $N$ lamps $(x_1, y_1), \\cdots, (x_N, y_N)$ are on, and all other lamps are off. Find $X$.\n\n## Constraints\n\n*   $1 \\leq N \\leq 10^5$\n*   $-10^{17} \\leq x_i, y_i \\leq 10^{17}$\n*   $(x_i, y_i)$ are pairwise distinct.\n*   The input is consistent with the statement, and you can uniquely determine $X$.\n\n## Input\n\nInput is given from Standard Input in the following format:\n\n$N$\n$x_1$ $y_1$\n$:$\n$x_N$ $y_N$\n\n[samples]","is_translate":false,"language":"English"}],"meta":{"iden":"wtf19_c1","tags":[],"sample_group":[["4\n-2 1\n-2 2\n0 1\n1 0","\\-1\n\nThe following picture shows one possible sequence of operations:\n![image](https://img.atcoder.jp/wtf19/cff6dc4d81e995e9300ccbaca5bf85de.png)"]],"created_at":"2026-03-03 11:01:13"}}