Probability of two people sharing a birthday is 1/365.
For three people, there are three ways of getting a shared birthday
AB, AC, or BC
For four people, there are 6 ways.
For five, there are 10.
The progression goes 0,1,3,6,10,15,21,28,36,45,55,66,78,91,105,120,136,153, 171, 190
Half of 365 is...