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

class kick {
    
    void solve(BufferedReader br, PrintWriter out) throws IOException {
        String[] str = br.readLine().trim().split(" ");
        int p = Integer.parseInt(str[0]);
        int d = Integer.parseInt(str[1]);
        int m = Integer.parseInt(str[2]);
        long s = Integer.parseInt(str[3]);
        int x = (p-m)/d;
        if(s<p) {
            System.out.println(0);
            return;
        }
        if(s==p) {
            System.out.println(1);
            return;
        }
        long sum = (x+1)*(2*p-x*d)/2;
        if(sum==s) {
            out.println(x+1);
            return;
        }
        if(sum>s) {
            long k = 0;
            for(int i=0; i<=x; i++) {
                k += p - i*d;
                if(s-k<d) {
                    System.out.println(i);
                    return;
                }
            }
            return;
        }
        s -= sum;
        x += s/m;
        System.out.println(x+1);
    }
    
    
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        PrintWriter out = new PrintWriter(System.out);
        int t = 1;//Integer.parseInt(br.readLine().trim());
        while(t-->0) {
            new kick().solve(br, out);
        }
        out.close();
        br.close();
    }
}