def read_data():
    N = int(input())
    ab = []
    for i in range(N):
        a, b = map(int, input().split())
        ab.append(a + 4 * b)
    return N, ab

def solve(N, ab):
    max_ab = max(ab)
    min_c = 0
    for a4b in ab:
        c2 = max_ab - a4b
        if c2 % 2:
            return -1
        else:
            min_c += c2 // 2
    return min_c

N, ab = read_data()
print(solve(N, ab))