{"problem":{"name":"Derangement","description":{"content":"You are given a permutation $p_1,p_2,...,p_N$ consisting of $1$,$2$,..,$N$. You can perform the following operation any number of times (possibly zero): Operation: Swap two **adjacent** elements in th","description_type":"Markdown"},"platform":"AtCoder","limit":{"time_limit":2000,"memory_limit":262144},"difficulty":"None","is_remote":true,"is_sync":true,"sync_url":null,"sign":"arc082_b"},"statements":[{"statement_type":"Markdown","content":"You are given a permutation $p_1,p_2,...,p_N$ consisting of $1$,$2$,..,$N$. You can perform the following operation any number of times (possibly zero):\nOperation: Swap two **adjacent** elements in the permutation.\nYou want to have $p_i ≠ i$ for all $1≤i≤N$. Find the minimum required number of operations to achieve this.\n\n## Constraints\n\n*   $2≤N≤10^5$\n*   $p_1,p_2,..,p_N$ is a permutation of $1,2,..,N$.\n\n## Input\n\nThe input is given from Standard Input in the following format:\n\n$N$\n$p_1$ $p_2$ .. $p_N$\n\n[samples]","is_translate":false,"language":"English"}],"meta":{"iden":"arc082_b","tags":[],"sample_group":[["5\n1 4 3 5 2","2\n\nSwap $1$ and $4$, then swap $1$ and $3$. $p$ is now $4,3,1,5,2$ and satisfies the condition. This is the minimum possible number, so the answer is $2$."],["2\n1 2","1\n\nSwapping $1$ and $2$ satisfies the condition."],["2\n2 1","0\n\nThe condition is already satisfied initially."],["9\n1 2 4 9 5 8 7 3 6","3"]],"created_at":"2026-03-03 11:01:13"}}