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