def main():
    N = int(input())
    ans = []
    for i in range(N):
        n, m = [int(i) for i in input().split()]
        ans.append(m-n)

    if has_negative(ans) or len(set(ans)) != 1:
        print(-1)
    else:
        print(ans[0])

# メソッド名が適切でない。正の数でないものが含まれていたらTrue
def has_negative(values):
    negative = False
    for value in values:
        if value <= 0:
            negative = True
    return negative



if __name__ == '__main__':
    main()