Before revision:  We first propose a 4-approximation algorithm in O(m^{7k}n\log k) time for this problem , where k is a positive integer  .
After revision:  We first propose a 4-approximation algorithm in O(m^{7k}n\log k) time for this problem and then show that the running time can be improved by a multiplicative factor of m^k, where a positive integer .