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

m = max(code)

result = 0
perfect = 1
for n in code:
    d = m - n
    if d % 2:
        perfect = 0
        break
    result += d // 2
print(result if perfect else -1)