/*
ANALYSIS:

*/
#include <iostream>
#include <cstring>
#include <string>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <queue>
#include <stack>
#include <ctime>
#include <random>
#define int long long
#define i32 signed
#define endl '\n'
using namespace std;
const int MaxN=5;
int sp,wf,ma,q,hsp,hwf,status,ans;
//struct node{int a,b,c,d;node(int A,int B,int C,int D){a=A,b=B,c=C,d=D;}node(){}
//bool operator<(const node& b)const{if(this->a!=b.a)return this->a<b.a;if(this->b!=b.b)return this->b<b.b;if(this->c!=b.c)return this->c<b.c; return this->d<b.d;}};
//set<node>S;
void solution()
{
	//sp=rand()%20+1;wf=rand()%20+1;ma=rand()%4+1;q=rand()%4;
	cin>>sp>>wf>>ma>>q;
	//cout<<"问题"<<endl;cout<<sp<<' '<<wf<<' '<<ma<<' '<<q<<endl;
	hsp=hwf=0;
	status=1;//out; 2 is home
	ans=0;
	//cout<<"过程"<<endl;
	while(sp!=0)
	{
		ans++;
		if(status==1)
		{
			if(sp<=ma)
			{
				hsp+=sp;
				sp=0;
				break;
			}
			for(int cnt=1;cnt<=ma;cnt++)
			{
				if(sp+q<=wf&&sp!=0)
				{
					if(wf==0)continue;
					wf--;
					hwf++;
				}
				else if(sp+q>wf)
				{
					if(sp==0)continue;
					sp--;
					hsp++;
				}
			}
			
			if(wf>sp+q&&sp!=0)
			{
				cout<<-1<<endl;
				return;
			}

			status=2;
		}
		else
		{
			for(int cnt=1;cnt<=ma;cnt++)
			{
				if(hsp+q<hwf&&hsp!=0)
				{
					if(hwf==0)continue;
					hwf--;
					wf++;
				}
				else
				{
					;
				}
			}
			if(hwf>hsp+q&&hsp!=0)
			{
				cout<<-1<<endl;
				return;
			}
			status=1;
		}

		if(ans>1000005)
		{
			cout<<-1<<endl;
			return;
		}
		//cout<<sp<<' '<<wf<<' '<<hsp<<' '<<hwf<<' '<<status<<endl;
	}
	//cout<<"答案"<<endl;
	cout<<ans<<endl;
}

i32 main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);cout.tie(0);
	//preprocess();
	int T=1;
	//cin>>T;
	srand(time(0));
	while(T--)
	{
		solution();
	}
	return 0;
}