---

#include <iostream>
#include <cmath>
using namespace std;

double log2(double x) { return log(x) / log(2); }

int main() {
    double x, y;
    
    cout << "x = "; cin >> x;
    cout << "y = "; cin >> y;
    
    try {
        if (x - y <= 0) throw "Аргумент логарифма должен быть > 0";
        
        double result = log2(x - y);
        cout << "log2(" << x << "-" << y << ") = " << result << endl;
    }
    catch (const char* msg) {
        cerr << "Ошибка: " << msg << endl;
    }
    
    return 0;
}