#include <bits/stdc++.h>

const int N = 105;

int f[N][N][2];
bool vis[N][N][2];
int x, y, p, q;

void chkMin(int &_, int __) {
    _ = std::min(_, __);
}

int id(int i, int j, int cur) {
    return i * (y + 1) + j + 1 + cur * (x + 1) * (y + 1);
}

int main() {
    std::cin >> x >> y >> p >> q;
    const int tot = 2 * (x + 1) * (y + 1);
    std::bitset<21001> e[tot + 1];
    for (int i = 0; i <= x; ++i) {
        for (int j = 0; j <= y; ++j) {
            for (int cur = 0; cur < 2; ++cur) {
                for (int ii = 0; ii <= i && ii <= p; ++ii) {
                    for (int jj = 0; jj <= j && ii + jj <= p; ++jj) {
                        if (i - ii + q >= j - jj || i - ii == 0) {
                            // printf("%d %d  %d %d  %d\n", i, j, ii, jj, cur);
                            e[id(i, j, cur)][id(x - i + ii, y - j + jj, cur ^ 1)] = 1;
                        }
                    }
                }
            }
        }
    }
    std::vector<int> d(tot + 1, 0x3f3f3f3f);
    std::vector<bool> vis(tot + 1);
    d[id(x, y, 0)] = 0;
    for (int i = 1; i < tot; ++i) {
        int u = 0;
        for (int j = 1; j <= tot; ++j) {
            if (!vis[j] && d[j] <= d[u]) {
                u = j;
            }
        }
        vis[u] = true;
        for (int v = 1; v <= tot; ++v) {
            if (e[u][v]) {
                d[v] = std::min(d[v], d[u] + 1);
            }
        }
    }
    int min = 0x3f3f3f3f;
    for (int i = 0; i <= y; ++i) {
        min = std::min(min, d[id(x, i, 1)]);
    }
    if (min == 0x3f3f3f3f) {
        std::cout << "-1\n";
    } else {
        printf("%d\n", min);
    }
    return 0;
}