#include <stdio.h>
#include <math.h>

int main() {
  int n = 0, p = 0, min;

  scanf("%d", &n);
  scanf("%d", &p);

  min = (n / 2) - (p / 2);
  
  if (min > p / 2)
    min = p / 2;
  printf("%d", min);
  return 0;
}