n = int(input())
c = []
for i in range(n):
    a, b = map(int, input().split())
    t = a + 4 * b
    c.append(t)

ans = 0
maxc = max(c)

for i in c:
    diff = maxc - i
    if diff % 2 == 0:
        ans += diff // 2
    else:
        ans = -1
        break

print(ans)