#The solution in Python3 is as follows:
n=int(input())
p=int(input())
if (n%2!=0 and p%2!=0) or (n%2!=0 and p%2==0) or (n%2==0 and p%2==0):
print(min(p//2,(n-p)//2))
elif n%2==0 and p%2!=0:
print(min(p//2,((n-p)//2)+1))
#Keep visiting for more solutions of HackerRank problems in Python 3.Thank you!!!
