n = int(input())
w = [list(map(int, input().split())) for _ in range(n)]
q = list()
for x in w:
    q.append(x[0] + x[1]*4)
m = max(q)
if all([a % 2 == 0 for a in q]) or all([a % 2 != 0 for a in q]):
    c = 0
    for i in q:
        c = c + (m - i)
    print(c//2)
else:
    print(-1)