n = int(input())
code = [int(a) + (4 * int(b)) for a, b in map(lambda x: x.split(), iter(input, ''))]
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)