Project Euler problem#15

#python program to find the lattice path of 20x20 grid
a=[1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1]
for j in range(1,21):
    i=1
    while i <21 :
        a[i] = int(a[i-1])+int(a[i])
        i+=1
print (a[20])