From hospital records, we identify the median birthweight of healthy infants (where the median is the weight such that half of the healthy children have birthweights above the median and half have birthweights below the median). In a group of 20 HIV infected children, we observe 17 to have birthweights below the median from healthy children. What is probability to observe this sample if HIV infected children have normal birthweight? (Note: The probability you calculate is the p-value of the sign test).