#include<iostream>
#include<cstdio>
#include<algorithm>
#include<queue>
using namespace std;

#define MAXN 100
#define INF 1000000000
#define LL long long
#define Pr pair<int,int>
#define X first
#define Y second

LL read(){
    LL x=0,F=1;char c=getchar();
    while(c<'0'||c>'9'){if(c=='-')F=-1;c=getchar();}
    while(c>='0'&&c<='9'){x=x*10+c-'0';c=getchar();}
    return x*F;
}

struct node{
    int op,a,b;
};

int x,y,p,q,ans;
int f[2][MAXN+5][MAXN+5];
queue<node> Q;

int main(){
    y=read(),x=read(),p=read(),q=read();
    ans=INF;
    for(int i=0;i<=x;i++)
        for(int j=0;j<=y;j++)
        f[0][i][j]=f[1][i][j]=-1;
    f[0][x][y]=0;
    Q.push((node){0,x,y});
    while(!Q.empty()){
        node s=Q.front();
        Q.pop();
        int op=s.op,a=s.a,b=s.b;
        for(int i=0;i<=a;i++)
            for(int j=0;j<=b;j++)
            if((a-i)+(b-j)<=p){
                if(j!=0&&i>j+q){
                    //printf("**%d %d %d\n",op,a,b);
                    continue;
                }
                if(f[op^1][x-i][y-j]==-1){
                    f[op^1][x-i][y-j]=f[op][a][b]+1;
                    //printf("%d %d %d->%d %d %d\n",op,a,b,op^1,x-i,y-j);
                    Q.push((node){op^1,x-i,y-j});
                }
            }
    }
    for(int i=0;i<=x;i++)
    if(f[1][i][y]!=-1)
        ans=min(ans,f[1][i][y]);
    if(ans==INF)puts("-1");
    else printf("%d\n",ans);
}