How to Solve this question.

How to approach below problem?

Given an array of integer, the squared sum is defined as the sum of squares of all elements. Given that the cost of changing element x of array to y is (x-y)^2 and element once changed cannot be changed again. Change the given array elements to get the desired squared sum.
For example :

Suppose array is {3,3,1} and desired squared sum is 6. Than we can change 3 to 1 and next 3 to 2 so the array becomes {1,2,1} or {2, 1, 1} and 1^2 + 2^2 + 1^2 = 6 i.e desired sum and the cost of change is (3-2)^2 + (3-1)^2 = 5.

Please mention the input and output constraints so that we can think in that direction.