#include<bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
#define For(i,j,k) for (int i=(j);i<=(k);i++)
#define rof(i,j,k) for (int i=(j);i>=(k);i--)
#define ull unsigned long long
#define lowbit(x) ((x)&(-(x)))
#define PII pair<int,int>
#define int long long
#define endl "\n"
#define ls rt<<1
#define rs rt<<1|1
typedef long long ll;
const int mod = 998244353;
const double eps = 1e-9;
const int N=207;
int n,m,p,q;
int a[N];
int ans=0x3f3f3f3f;
struct node{
    int sh;
    int wol;
    int no;
};
queue<node> Q;
int dp[207][207][2];
void solve(){
	cin>>n>>m>>p>>q;
    Q.push({n,m,0});
    memset(dp,-1,sizeof dp);
    dp[n][m][0]=0;
    while(!Q.empty()){
        int sh=Q.front().sh;
        int wl=Q.front().wol;
        int no=Q.front().no;
        Q.pop();
        if(no==0){
            for(int i=0;i<=sh;i++){
                for(int j=0;j<=wl;j++){
                    if((i+j)<=p&&dp[sh-i][wl-j][1]==-1&&((sh-i+q)>=(wl-j)||sh-i==0)){
                        dp[sh-i][wl-j][1]=dp[sh][wl][0]+1;
                        Q.push({sh-i,wl-j,1});
                    }
                }
            }
        }else{
            for(int i=0;i<=n-sh;i++){
                for(int j=0;j<=m-wl;j++){
                    if((i+j)<=p&&dp[sh+i][wl+j][0]==-1&&((n-sh-i+q)>=(m-wl-j)||(n-sh-i)==0)){
                        dp[sh+i][wl+j][0]=dp[sh][wl][1]+1;
                        Q.push({sh+i,wl+j,0});
                    }
                }
            }
        }
    }
    for(int i=0;i<=m;i++){
        if(dp[0][i][1]<ans&&dp[0][i][1]!=-1) ans=dp[0][i][1];
    }
    if(ans==0x3f3f3f3f) cout<<"-1"<<endl;
    else cout<<ans<<endl;
}
signed main(){
	int t=1;
	//cin>>t;
	while(t--){
		solve();
	}
}