输入一个正整数nnn，计算
1+(1−3)/1+(1−3+5)/1+(1−3+5−...+2n−1)/11+(1-3)/1+(1-3+5)/1+(1-3+5-...+2n-1)/11+(1−3)/1+(1−3+5)/1+(1−3+5−...+2n−1)/1的值
想了好久了，都要被逼疯了
