def main():
    n = int(input())
    s = set()
    for _ in range(n):
        x, y = map(int, input().split())
        s.add(y - x)

    if len(s) == 1:
        ans = s.pop()
        if ans <= 0:
            print(-1)
        else:
            print(ans)
    else:
        print(-1)
    
    
if __name__ == '__main__':
    main()