#include <iostream>
using namespace std;

int main() {
    double ab;
    double bc;
    double weight;
    cout << "Distance A-B: ";
    cin >> ab;
    cout << "Distance B-C: ";
    cin >> bc;
    cout << "Cargo weight: ";
    cin >> weight;

    double fuel;
    if (weight <= 500) fuel = 1;
    else if (weight <= 1000) fuel = 4;
    else if (weight <= 1500) fuel = 7;
    else if (weight <= 2000) fuel = 9;
    else {
        cout << "The airplane cannot lift such a load!" << endl;
        return 0;
    }

    double nAB = ab * fuel;
    double nBC = bc * fuel;

    if (nAB > 300) {
        cout << "It is impossible to reach point B by flight." << endl;
    }
    else if (nBC > 300) {
        cout << "It is impossible to fly from B to C." << endl;
    }
    else {
        double fill = max(0.0, nBC - (300 - nAB));
        cout << "Minimum refuel at B: " << fill << " liters." << endl;
    }
    return 0;
}