代入f1=-1,fn=m：
1-m+n=-1 ==> m-n=2
n^2-mn+n=m ==> (n+1)(n-m)=0 ==> n+1=0
n=-1,m=1
f(x)=x^2-x-1
f(-1)=1+1-1=1
f(f(-1))=f(1)=-1
f(f(x))=(x^2-x-1)^2-(x^2-x-1)-1=x^4-2x^3-2x^2+3x+1