洛谷
令(n=2000),(a_1,a_2,...,a_{1998}=0,a_{1999}=-d,a_{2000}=x-d) 则(2000x-(x+d)=kLongrightarrow 1999x=k+d) (x=frac{k+d}{1999},d=1999-k\%1999)