Baekjoon #22864

import java.util.*;
import java.io.*;

public class Main {
    public static void main(String[] args) {
        FastReader rd = new FastReader();

        int A = rd.nextInt();
        int B = rd.nextInt();
        int C = rd.nextInt();
        int M = rd.nextInt();

        int time = 0;
        int tired = 0; // 피로도
        int work = 0;

        while(time < 24) { // 24시간 동안 일
            if(can_work(A, tired, M)) { // 일을 할 수 있을 때
                work += B;
                tired += A;
            }
            else { // 일을 할 수 없을 때 => 쉰다.
                tired -= C;
                if(tired < 0) tired = 0; // 피로도가 0보다 작아지지 않게
            }
            time++;
        }
        System.out.print(work);
    }

    static boolean can_work(int A, int tired, int M) {
        if(tired + A <= M) return true;
        else return false;
    }

    static class FastReader {
        BufferedReader br;
        StringTokenizer st;

        public FastReader() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String next() {
            while(st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                }
                catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt() { return Integer.parseInt(next()); }
        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            }
            catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }
}