{"raw_statement":[{"iden":"statement","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 $S_n$。"},{"iden":"input","content":"输入为一行，含一个正整数 $n$。"},{"iden":"output","content":"输出为一行，为 $S_n$ 的值，结果保留小数点后 $4$ 位小数。"},{"iden":"note","content":"$1\\leq n\\leq 1000$。"}],"translated_statement":null,"sample_group":[["2","0.5000"]],"show_order":[],"formal_statement":null,"simple_statement":null,"has_page_source":false}