#include <stdio.h>
int main(){
double eps;
double r;
int i,s=1;
double sum=0;
if (scanf("%lf",&eps)!=EOF){
//if (eps<0.003){printf("%lf",eps);return 0;}
for (i=0;1;i++){
r = 1.0/(3*i+1);
sum+=r*s;
//printf("i=%d s=%d r=%f eps=%f sum=%f\n",i,s,r,eps,sum);
s*=-1;
if (r<=eps){break;}
}
printf("sum = %.6f",sum);
}else{
printf("input eps wrong");
}
return 0;
}
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