#include <iostream>
signed main()
{
double a, b, x;
std::cin >> a >> b;
x = (a + b - 3.0) / 2.0;
std::cout << x;
return 0;
}
#include <iostream>
signed main()
{
double a, b, x;
std::cin >> a >> b;
x = (a + b - 3.0) / 2.0;
std::cout << x;
return 0;
}