Center of symmetry

Given is a set of n points with integer coordinates. Your task is to decide whether the set has a center of symmetry. A set of points S has the center of symmetry if there exists a point s (not necessarily in S) such that for every point p in S there exists a point q in S such that p − s = s − q. Input The first line of input contains a number c giving the number of cases that follow. The first line of data for a single case contains number 1 ≤ n ≤ 10000. The subsequent n lines contain two integer numbers each which are the x and y coordinates of a point. Every point is unique and we have that −10000000 ≤ x, y ≤ 10000000. Output For each set of input data print ‘yes’ if the set of points has a center of symmetry and ‘no’ otherwise. Sample Input 1 8 1 10 36 68 62 3 -4 10 -2 -2 -2 4 Sample Output yes