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

s = max(rec)
num = 0
for i in range(n):
    if (s - rec[i]) % 2 == 0:
        num += (s - rec[i]) // 2

    else:
        num = -1
        break

print(num)