Stuck with the Question

Can someone help me with this question?

Given an array A. Each entry of A is of form {cost[i] , value[i]}. Also we have a condition that if (cost[i]<cost[j]) then (value[i]<value[j]). We need to find a subset of A of size k, which has a total value(sum of the value of all k elements) greater than or equal to (n-k) and has a minimum cost(cost of a subset is defined as the sum of the cost of all k elements).

We need to return the minimum cost.

Thank you