#include <bits/stdc++.h>
#define rep(i,a,b) for(int i=(a);i<(b);++i)
using namespace std;
using pii = pair<int,int>;
using ll = long long;
using ull = unsigned long long;
using lll = __int128;
using pll = pair<ll,ll>;
using vi = vector<int>;

#define fr first 
#define sc second

using ld = long double;

#define all(x) (x).begin(), (x).end()

const int maxn = 1e2+10, inf = 1e9+100;
const ll linf = 8e18+100;
const int mod = 998244353;



// #define cerr if(0) cerr

int dp[maxn][maxn][2];
void solve () {
	int x,y,p,q;
	cin >> x >> y >> p >> q;

	using T = tuple<int,int,bool>;
	queue<T> pq;
	memset(dp,-1,sizeof(dp));
	dp[0][0][0]=0;
	pq.push({0,0,0});
	while(pq.size()){
		auto [ox,oy,sd] = pq.front();
		// cerr << ox << " " << oy << " " << sd << endl;
		int sn = sd==0?1:-1;
		pq.pop();
		for(int i=0;i<=p;i++)for(int j=0;i+j<=p;j++){
			int nox = ox + i * sn, noy = oy + j * sn;
			bool nsd = sd ^ 1;
			if(nox<0||nox>x||noy<0||noy>y)continue;
			if(dp[nox][noy][nsd]!=-1)continue;
			bool bad = 0;
			if(!sd){
				if((x-ox-i)+q<(y-oy-j)&&x-ox-i>0)bad = 1;
			}
			else {
				if((ox-i)+q<(oy-j)&&ox-i>0)bad = 1;
			}
			if(bad)continue;
			dp[nox][noy][nsd] = dp[ox][oy][sd]+1;
			if(nox == x) {
				cout << dp[nox][noy][nsd] << "\n";
				return;
			}
			pq.push({nox,noy,nsd});
		}
	}
	cout << "-1\n";
}


int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout<<fixed;
  cout.precision(20);
	// int tc;
	// cin >> tc;
	// while(tc--)
  solve();
}