N = int(input())
box = []
mx = 0
space = 0
error = 0
for i in range(N):
    X = [int(i) for i in input().split()]
    box.append(X[0]+X[1]*4)
mx = max(box)
box.remove(max(box))
for i in range(len(box)):
    space += (mx-box[i])/2
    if (mx-box[i])%2 != 0:
        error = 1
if error == 1:
    print(-1)
else:
    print(int(space))