#Lattice paths
#Problem 15
#
#Starting in the top left corner of a 2x2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.
#__   _    _       
#  |   |_   |   |   |_    |__ 
#  |     |  |_  |__   |_     |
#
#How many such routes are there through a 20x20 grid?
#
#Solution:

a = 1
b = 1
for i in range(21,41):
    a *= i
    b *= i-20
print( a//b )