>23
#include <stdio.h>
int main()
{
double sum = 0.0;
double e = 0.00001;
int x = 1;
double a;
while (1) {
a = 1.0 / x; sum += a;
if (a < e) break;
x *= 3;
}
printf("%.15lf\n", sum);
return 0;
}