C++ :

#include <iostream>
using namespace std;
int main() {
    int books, cost = 0;
    cin >> books;
    for (int i = 1; i <= books; i++) {
        if (i > 3) {
            cost += 5 * 0.8; // 8折
        } else {
            cost += 5;
        }
    }
    cout << cost;
    return 0;
}