#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>
#include <cmath>
#define FOR() ll le=e[u].size();for(ll i=0;i<le;i++)
#define QWQ cout<<"QwQ\n";
#define ll long long
#include <vector>
#include <queue>
#include <map>

using namespace std;
const ll N=501010;
const ll qwq=303030;
const ll inf=0x3f3f3f3f;

ll T;
int X,Y,Q,P;
struct D{
    int di,x,y,cl;
};
queue <D> q;
int dis[111][111][2];

inline ll read() {
    ll sum = 0, ff = 1; char c = getchar();
    while(c<'0' || c>'9') { if(c=='-') ff = -1; c = getchar(); }
    while(c>='0'&&c<='9') { sum = sum * 10 + c - '0'; c = getchar(); }
    return sum * ff;
}

inline bool check(int x,int y,int cl) {
    if(cl==1) {
        return (Y-y)<=((X-x)+P) || x==X;
    }
    else {
        return y<=(x+P) || x==0;
    }
}

int main() {
    X = read(); Y = read(); Q = read(); P = read();
    memset(dis,0x3f,sizeof(dis));
    dis[0][0][0] = 0; q.push({0,0,0,0});
    while(!q.empty()) {
        D now = q.front(); q.pop();
        // cout<<now.di<<" : "<<now.x<<" "<<now.y<<" "<<now.cl<<"\n";
        if(now.cl==0) {
            for(int i=0;i<=X-now.x;i++) {
                for(int j=0;j<=Y-now.y;j++) {
                    if(j+i>Q) break;
                    if(!check(now.x+i,now.y+j,1)) continue;
                    // cout<<" i = "<<i<<" j = "<<j<<endl;
                    if(now.x+i==X) { cout<<now.di+1; return 0; }
                    if(dis[now.x+i][now.y+j][1] > now.di+1)
                    q.push({now.di+1,now.x+i,now.y+j,1}), dis[now.x+i][now.y+j][1] = now.di+1;
                }
            }
        }
        else {
            for(int i=0;i<=now.x;i++) {
                for(int j=0;j<=now.y;j++) {
                    if(j+i>Q) break;
                    if(!check(now.x-i,now.y-j,0)) continue;
                    if(dis[now.x-i][now.y-j][0] > now.di+1)
                    q.push({now.di+1,now.x-i,now.y-j,0}), dis[now.x-i][now.y-j][0] = now.di+1;
                }
            }
        }
    }
    cout<<-1;
    return 0;
}