var('x,y')
f(x,y)=x^20+y^20
p(a,b)=0
for m in range(11):
    p(a,b)=p(a,b)+f(a,0)*b^m
    f(x,y)=expand((f(x,y)-f(x+y,0))/x/y) 
print (expand(p(x+y,x*y)))