C++ :

#include<iostream>
#include<cstdio>
using namespace std;
int main()
{
	int n;
	double sum=0;
	int i;

	cin>>n;//前n项
	for(i=1;i<=n;i++)//数列规律
	{		
		if(i%2==0)	sum-=1.0/i;
		else sum+=1.0/i;
	}
	printf("%.4lf\n",sum);
	return 0;
}