/*
ANALYSIS:

*/
#include <iostream>
#include <cstring>
#include <string>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <queue>
#include <stack>
#include <ctime>
#include <random>
#define int long long
#define i32 signed
#define endl '\n'
using namespace std;

const int N = 105;
struct Node {
	int sheep, wolf, bank;
};
int boat, gap;
vector<Node> G[2][N][N];
bool vis[N][N][2];

bool isValid(int p, int q) {
	if (p + gap >= q || p == 0) return true;
	return false; 
}

void solution()
{
	int sheep, wolf;
	cin >> sheep >> wolf >> boat >> gap;
	for (int i = 0; i <= sheep; i++)
		for (int j = 0; j <= wolf; j++)
			for (int p = 0; p <= sheep; p++)
				for (int q = 0; q <= wolf; q++) {
					if ((i-p)*(j-q)<0) continue;
					if (abs(i-p)+abs(j-q)>boat) continue;
					if (i>=p && j>=q && isValid(sheep - i, wolf - j) && isValid(p, q)) G[0][i][j].push_back({p, q, 1});
					if (p>=i && q>=j && isValid(i, j) && isValid(sheep - p, wolf - q)) G[0][p][q].push_back({i, j, 1});
					if (i<=p && j<=q && isValid(i, j) && isValid(sheep - p, wolf - q)) G[1][i][j].push_back({p, q, 0});
					if (p<=i && q<=j && isValid(sheep - i, wolf - j) && isValid(p, q)) G[1][p][q].push_back({i, j, 0});
				}
	queue<pair<Node, int> > que;
	que.push({{sheep, wolf, 0}, 0});
	vis[sheep][wolf][0] = true;
	bool flag = false;
	while (!que.empty()) {
		auto x = que.front();
		Node u = x.first;
		if (u.sheep == 0) {
			cout << x.second << endl;
			flag = true;
			break;
		} 
		que.pop();
		for (const auto& v : G[u.bank][u.sheep][u.wolf]) {
			if (!vis[v.sheep][v.wolf][v.bank]){
				vis[v.sheep][v.wolf][v.bank]=true;
				que.push({v, x.second + 1});
			}
		}
	}
	if (!flag) cout << -1 << endl;
}

i32 main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);cout.tie(0);
	//preprocess();
	int T=1;
	// cin>>T;
	while(T--)
	{
		solution();
	}
	return 0;
}