题解（超简单

#include<iostream>
using namespace std;
int main(){
    double sum=0;
    int n;
    cin>>n;
    for(int i=1;i<=n;i++){
        double x=1.0/i;
        if(i%2==0)sum-=x;
        else sum+=x;
    }
    printf("%.4f",sum);
    return 0;
}