include <bits/stdc++.h>

using namespace std; int main() {

long a,b,n,l=0;
cin>>a>>b;
if(a%2==0 && b%2==0)
    cout << (long long)(max(a,b)-min(a,b))/2-1;
else
    cout << (long long)(max(a,b)-min(a,b))/2;

}