题解

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