#include <iostream>
#include <cmath>
using namespace std;
int main() 
{
 double sum, eps, prev,next,dif;
 int i;
 eps=1e-10;
 prev=1; next=-1/3.0;
 i=2; sum=1;
 dif=abs(next-prev);
 while (dif>eps)
 {
     sum=sum+next;
     prev=next;
     next=prev*(-1)/((2*i+1)*i);
     i=i+1;
     dif=abs(prev-next);
 }
 cout << "sum="<< sum << "\n" << endl;
 return 0;
}