The answer of this problem is given in [the paper](file:///D:/%E6%88%91%E7%9A%84%E5%9D%9A%E6%9E%9C%E4%BA%91/%E8%AE%A8%E8%AE%BA%E7%8F%AD%E8%AE%B2%E8%AF%BE%E5%86%85%E5%AE%B9/Quantum%20affine%20algebras.pdf) by Chari and Pressley (the Corollary on Page 272).