package com.jd;

import java.io.BufferedInputStream;

import java.util.HashMap;

import java.util.Scanner;

public class Main {

public static void main(String[] args) {

Scanner in = new Scanner(new BufferedInputStream(System.in));

int tankNum = in.nextInt();

int b = in.nextInt(); // 单个碉堡的生命值

int destroy = in.nextInt(); // 单个碉堡的伤害

int d = in.nextInt();

int result = 0;

HashMap<Integer, Integer> diaobao = new HashMap<>();

for (int i = 1; i <= d; i++) {

diaobao.put(i, b); // 将碉堡和生命值放入map

}

while (tankNum >= 0 && diaobao.size() != 0) {

result++;

// 回合1, 坦克进攻

int tempTanke = tankNum;

while (tempTanke >= b && !diaobao.isEmpty()) {

diaobao.remove(diaobao.size() - 1);

tempTanke -= b;

}

if (diaobao.isEmpty()) break;

int curTankeCount = 0;

for (Integer has : diaobao.keySet()) {

curTankeCount += diaobao.get(has);

}

if (tempTanke <= curTankeCount && !diaobao.isEmpty()) {

int temp = curTankeCount - tempTanke;

if (temp <= 0) {

for (Integer key : diaobao.keySet()) {

diaobao.remove(key);

}

} else {

for (Integer key : diaobao.keySet()) {

diaobao.put(key, temp);

}

}

}

// 回合2, 碉堡进攻

if (!diaobao.isEmpty()) {

int curDiaoBao = diaobao.size();

tankNum -= destroy * curDiaoBao;

}

}

if (tankNum >= 0) {

System.out.println(result);

}

if (tankNum < 0 && !diaobao.isEmpty()) {

System.out.println(-1);

}

}

}