#include<bits/stdc++.h>
using namespace std;
signed main(){
double a,b;
cin>>a>>b;
cout<<(a-b)/3+b<<'\n';
return 0;
}
I2luY2x1ZGU8Yml0cy9zdGRjKysuaD4KIAp1c2luZyBuYW1lc3BhY2Ugc3RkOwogCgoKc2lnbmVkICBtYWluKCl7CiAKICAgICBkb3VibGUgYSxiOwogICAgY2luPj5hPj5iOwogICAgY291dDw8KGEtYikvMytiPDwnXG4nOwoKICAgIHJldHVybiAwOwp9CgoK