#include <stdio.h>
int main(void)
{
int n;
double s,x;
for (s=0,n=0; scanf("%lf",&x)!=EOF; n++) {/*數字輸入完成後另起壹行，按CTRL+Z或者CTRL+D（LINUX系統）輸入EOF並按回車結束輸入*/
s+=1/x;
}
printf("%lf",n/s);
return 0;
}
//---------------------------------------------------------------------------