{"problem":{"name":"计算分数加减表达式的值","description":{"content":"$S_n=\\frac{1}{1}-\\frac{1}{2}+\\frac{1}{3}-\\frac{1}{4}+\\frac{1}{5}-\\frac{1}{6}+...+(-1)^{n-1}\\times\\frac{1}{n}$。 输入一个整数 $n$，求 $S_n$。","description_type":"Markdown"},"platform":"Luogu","limit":{"time_limit":1000,"memory_limit":131072},"difficulty":{"LuoguStyle":"P1"},"is_remote":true,"is_sync":true,"sync_url":null,"sign":"LGB2070"},"statements":[{"statement_type":"Markdown","content":"$S_n=\\frac{1}{1}-\\frac{1}{2}+\\frac{1}{3}-\\frac{1}{4}+\\frac{1}{5}-\\frac{1}{6}+...+(-1)^{n-1}\\times\\frac{1}{n}$。\n\n输入一个整数 $n$，求 $S_n$。\n\n## Input\n\n输入为一行，含一个正整数 $n$。\n\n## Output\n\n输出为一行，为 $S_n$ 的值，结果保留小数点后 $4$ 位小数。\n\n[samples]\n\n## Note\n\n$1\\leq n\\leq 1000$。","is_translate":false,"language":"English"}],"meta":{"iden":"LGB2070","tags":["循环结构"],"sample_group":[["2","0.5000"]],"created_at":"2026-03-03 11:09:25"}}