#include<bits/stdc++.h>
using namespace std;
int dp[2][101][101];
int vis[2][101][101];
int main()
{
    int X,Y,p,q;
    cin>>X>>Y>>p>>q;
    function<int(int,int,int)>f=[&](int x,int y,int s)
    {
        // cerr<<x<<" "<<y<<" "<<s<<endl;
        if(x==0)return 0;
        if(vis[s][x][y])return dp[s][x][y];
        vis[s][x][y]=1;
        int&ans=dp[s][x][y];
        ans=1e9;
        if(s==0)
        {
            for(int i=0;i<=min(p,x);i++)
            {
                for(int j=0;j<=min(p-i,y);j++)
                {
                    if(x-i+p<y-j)continue;
                    ans=min(ans,f(x-i,y-j,1)+1);
                }
            }
        }
        else
        {
            for(int i=0;i<=min(p,X-x);i++)
            {
                for(int j=0;j<=min(p-i,Y-y);j++)
                {
                    if(X-x-i+p<Y-y-j)continue;
                    ans=min(ans,f(x+i,y+j,0)+1);

                }
            }
        }
        return dp[s][x][y]=ans;
    };
    int ans=f(X,Y,0);
    printf("%d\n",ans<1e9?ans:-1);
    return 0;
}