{"raw_statement":[{"iden":"statement","content":"有一个分数序列 $\\dfrac{q_{1}}{p_{1}},\\dfrac{q_{2}}{p_{2}},\\dfrac{q_{3}}{p_{3}},\\dfrac{q_{4}}{p_{4}},\\dfrac{q_{5}}{p_{5}},...$，其中 $q_{i+1}=q_{i}+p_{i}, p_{i+1}=q_{i},p_{1}=1,q_{1}=2$。比如这个序列前  $6$ 项分别是 $\\dfrac{2}{1},\\dfrac{3}{2},\\dfrac{5}{3},\\dfrac{8}{5},\\dfrac{13}{8},\\dfrac{21}{13}$。求这个分数序列的前 $n$ 项之和。"},{"iden":"input","content":"输入一行一个正整数 $n(n\\leq 30)$。"},{"iden":"output","content":"输出一行一个浮点数，表示分数序列前 $n$ 项的和，精确到小数点后 $4$ 位。"}],"translated_statement":null,"sample_group":[["2","3.5000"]],"show_order":[],"formal_statement":null,"simple_statement":null,"has_page_source":false}