#import pdb; pdb.set_trace()
import sys

def main():
    (n,m) = map(int,raw_input().split())
    
    if m == 0:
        print 0
    elif n < m:
        print 0
    elif n == m * 2 - 1:
        print max(n - 1, 0)
    else:
        print max(n - 2, 0)
        
if __name__ == '__main__':
    main()