#include <bits/stdc++.h>

using namespace std;

const int N = 110;
int n, m, p, q;
int f[N][N];

int main() {
    scanf("%d%d%d%d", &n, &m, &p, &q);
    memset(f, 0x3f, sizeof(f));
    f[n][m] = 0;
    for (int i = n; ~i; --i) {
        for (int j = m; ~j; --j) {
            if (i == 0 && j == 0) continue;
            for (int k = 0; k <= min(i, p); ++k) {
                int _k = min(j, p - k);
                for (int l = 0; l <= min(p, (m - (j - _k))); ++l) {
                    if ((i - k == 0 && j - _k + l == 0) || ((i - k + q >= j - _k || i - k == 0) && ((n - i + k) + q >= (m - (j - _k)) - l || (n - i + k) == 0))) {
                        f[i - k][j - _k + l] = min(f[i - k][j - _k + l], f[i][j] + 1);
                    }
                }
            }
        }
    }
    // cerr << f[0][100] << '\n';
    if (f[0][0] == 0x3f3f3f3f) {
        f[0][0] = -1;
    }
    printf("%d\n", f[0][0]);
    return 0;
}