http://en.wikipedia.org/wiki/Birthday_problem
for some reason, probably b/c of the random number generator, i get something around 11 to 13
> source('birthdays.R') ; main()
11 12 13 10 14 15 9 8 16 7 17
0.0928 0.0862 0.0857 0.0857 0.0784 0.0747 0.0742 0.0678 0.0623 0.0535 0.0427
6 18 19 5 20 4 21 3 22 23 2
0.0365 0.0355 0.0266 0.0265 0.0165 0.0161 0.0112 0.0088 0.0083 0.0032 0.0031
24 25 27 26 29 28
0.0021 0.0010 0.0002 0.0002 0.0001 0.0001