import sys

def solve():
    N, K = map(int, input().split())

    if K == 0 or K > N:
        print(0)
        return

    if N % 2 == 0:
        print(N - 2)
    else:
        if (N + 1) // 2 == K:
            print(N - 1)
        else:
            print(N - 2)

if __name__ == '__main__':
    solve()