from sys import stdin
readline = stdin.readline

def is_odd(i):
    return i % 2
def is_even(i):
    return i % 2 == 0

n = int(readline())
length = []
for _ in range(n):
    space, tab = map(int, readline().split())
    length.append(space + tab * 4)

if all(map(is_odd, length)) or all(map(is_even, length)):
    print((max(length) * len(length) - sum(length)) // 2)
else:
    print(-1)