#include<bits/stdc++.h>
using namespace std;
int x,y,p,q;
const int N=105;
int d[N][N][2];
const int inf=100005;
struct dt
{
    int x,y,o;
};
bool check(int nx,int ny,int o)
{
    return o==0?(x-nx+q>=y-ny||x-nx==0):(nx+q>=ny||nx==0);
}
int main()
{
    scanf("%d%d%d%d",&x,&y,&p,&q);
    for (int i=0;i<=x;++i)
        for (int j=0;j<=y;++j)
            for (int k=0;k<=1;++k)
                d[i][j][k]=inf;
    d[x][y][0]=0;
    queue<dt> q;
    q.push({x,y,0});
    while (!q.empty())
    {
        int nx=q.front().x;
        int ny=q.front().y;
        int o=q.front().o;
        // cout<<nx<<' '<<ny<<' '<<o<<' '<<d[nx][ny][o]<<endl;
        q.pop();
        if (o==1&&nx==0)
        {
            printf("%d\n",d[nx][ny][o]);
            return 0;
        }
        if (o==0)
        {
            for (int i=max(0,nx-p);i<=nx;++i)
                for (int j=0;j<=ny;++j)
                    if (nx-i+ny-j<=p&&d[i][j][1]==inf&&check(i,j,1))
                    {
                        d[i][j][1]=d[nx][ny][o]+1;
                        q.push((dt){i,j,1});
                    }
        }
        else
        {
            for (int i=nx;i<=min(x,nx+p);++i)
                for (int j=ny;j<=y;++j)
                    if (i-nx+j-ny<=p&&d[i][j][0]==inf&&check(i,j,0))
                    {
                        d[i][j][0]=d[nx][ny][o]+1;
                        q.push((dt){i,j,0});
                    }
        }
    }
    puts("-1");
    return 0;
}