n = int(input())
mx = 0
sm = 0
for i in range(n):
    a, b = map(int, input().split())
    w = a + 4 * b
    mx = max(mx, w)
    sm += w

if (n * mx - sm) % 2 == 0:
    print((n * mx - sm) // 2)
else:
    print(-1)