def solve():
    N = int(input())
    W = [0] * N
    for i in range(N):
        s, t = list(map(int, input().split()))
        W[i] = s + t * 4

    mi = min(W)
    ans = 0
    for i in range(N):
        n = W[i] - mi
        if n % 2 == 1:
            return -1
        ans += n // 2

    return ans


def main():
    print(solve())


if __name__ == '__main__':
    main()