C++解答

#include<iostream>
#include<cstdio>
using namespace std;
int main()
{
	double sum=0;
	int a=1,sign=1;
	while (1.0/a>=1e-6){
		sum+=sign*1.0/a;
		a+=2;
		sign*=-1;
	}
	printf("%.6lf\n",4*sum);
	return 0;
}