def BinarySearch_solve():
    n = int(input())
    k = int(input())
    left = -1   #red point - точно не решает
    right = n   #yellow point - точно решает
    while right > left+1:
        middle = (left + right)//2
        if (n+middle)>3*(k+middle):
            left = middle
        else:
            right = middle
    print(right)

BinarySearch_solve()