static void Main(string[] args)
{
int[] input = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray<int>();
List<int> result = new List<int>();
if (input[1] == 2)
Console.WriteLine(1);
else
{
int diffTemp = input[1] - (input[0] * 2);
for (int i = 0; i < input[0]; i++)
result.Add(2);
int j = 0;
while (diffTemp > 0 || result.Where(x => x == 3).Count() == result.Count)
{
if (j == result.Count)
j = 0;
result[j]++;
j++;
diffTemp--;
}
Console.WriteLine(result.Where(x => x == 2).Count());
}
Console.ReadLine();
}
