#include<bits/stdc++.h>

using namespace std;

typedef long long llint;
typedef pair <int, int> pi;

const int MAXN = 101;

int s, w, p, q;
short dist[MAXN][MAXN][MAXN][MAXN][2];

struct state {
    int s1, w1, s2, w2, t;
    state () {}
    state (int _s1, int _w1, int _s2, int _w2, int _t) {
        s1 = _s1; w1 = _w1; s2 = _s2; w2 = _w2; t = _t;
    }
};
deque <state> dq;

short bfs () {
    dist[s][w][0][0][0] = 0;
    dq.push_back(state(s, w, 0, 0, 0));
    while (!dq.empty()) {
        state C = dq.front();
        dq.pop_front();
        int d = dist[C.s1][C.w1][C.s2][C.w2][C.t];

        if (C.t == 1 && C.s2 == 0) return d;

        if (s + w - (C.s1 + C.w1 + C.s2 + C.w2) < p) {
            if (C.s1 > 0) {
                if (dist[C.s1 - 1][C.w1][C.s2][C.w2][C.t] == -1) {
                    dist[C.s1 - 1][C.w1][C.s2][C.w2][C.t] = d;
                    dq.push_front(state(C.s1 - 1, C.w1, C.s2, C.w2, C.t));
                }
            }
            if (C.w1 > 0) {
                if (dist[C.s1][C.w1 - 1][C.s2][C.w2][C.t] == -1) {
                    dist[C.s1][C.w1 - 1][C.s2][C.w2][C.t] = d;
                    dq.push_front(state(C.s1, C.w1 - 1, C.s2, C.w2, C.t));
                }
            }
        }
        if (C.s1 == 0 || C.w1 - C.s1 <= q) {
            if (dist[s - C.s1][w - C.w1][C.s1][C.w1][!C.t] == -1) {
                dist[s - C.s1][w - C.w1][C.s1][C.w1][!C.t] = d + 1;
                dq.push_back(state(s - C.s1, w - C.w1, C.s1, C.w1, !C.t));
            }
        }
    }
    return -1;
}

int main () {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    memset(dist, -1, sizeof dist);
    cin >> s >> w >> p >> q;
    cout << bfs();
    return 0;
}