#include <bits/stdc++.h>
#define _rep(i, x, y) for(int i = x; i <= y; ++i)
#define _req(i, x, y) for(int i = x; i >= y; --i)
#define _rev(i, u) for(int i = head[u]; i; i = e[i].nxt)
#define pb push_back
#define fi first
#define se second
#define mst(f, i) memset(f, i, sizeof f)
using namespace std;
#ifdef ONLINE_JUDGE
#define debug(...) 0
#else
#define debug(...) fprintf(stderr, __VA_ARGS__), fflush(stderr)
#endif
typedef long long ll;
typedef pair<int, int> PII;
namespace fastio{
    #ifdef ONLINE_JUDGE
    char ibuf[1 << 20],*p1 = ibuf, *p2 = ibuf;
    #define get() p1 == p2 && (p2 = (p1 = ibuf) + fread(ibuf, 1, 1 << 20, stdin), p1 == p2) ? EOF : *p1++
    #else
    #define get() getchar()
    #endif
    template<typename T> inline void read(T &t){
        T x = 0, f = 1;
        char c = getchar();
        while(!isdigit(c)){
            if(c == '-') f = -f;
            c = getchar();
        }
        while(isdigit(c)) x = x * 10 + c - '0', c = getchar();
        t = x * f;
    }
    template<typename T, typename ... Args> inline void read(T &t, Args&... args){
        read(t);
        read(args...);
    }
    template<typename T> void write(T t){
        if(t < 0) putchar('-'), t = -t;
        if(t >= 10) write(t / 10);
        putchar(t % 10 + '0');
    }
    template<typename T, typename ... Args> void write(T t, Args... args){
        write(t), putchar(' '), write(args...);
    }
    template<typename T> void writeln(T t){
        write(t);
        puts("");
    }
    template<typename T> void writes(T t){
        write(t), putchar(' ');
    }
    #undef get
};
using namespace fastio;
#define multitest() int T; read(T); _rep(tCase, 1, T)
namespace Calculation{
    const ll mod = 998244353;
    ll ksm(ll p, ll h){ll base = p % mod, res = 1; while(h){if(h & 1ll) res = res * base % mod; base = base * base % mod, h >>= 1ll;} return res;}
    void dec(ll &x, ll y){x = ((x - y) % mod + mod) % mod;}
    void add(ll &x, ll y){x = (x + y) % mod;}
    void mul(ll &x, ll y){x = x * y % mod;}
    ll sub(ll x, ll y){return ((x - y) % mod + mod) % mod;}
    ll pls(ll x, ll y){return ((x + y) % mod + mod) % mod;}
    ll mult(ll x, ll y){return x * y % mod;}
}
using namespace Calculation;
const int N = 105, inf = 0x3f3f3f3f;
int x, y, p, q, f[N][N], g[N][N];
bool check(int a, int b){
    return !a || b - a <= q;
}
struct node{
    int x, y, op;
};
int main(){
    read(x, y, p, q), mst(f, inf), mst(g, inf);
    f[x][y] = 0;
    queue<node> q; q.push({x, y, 0});
    while(q.size()){
        int i = q.front().x, j = q.front().y, op = q.front().op; q.pop();
        if(!op){
            _rep(a, 0, i) _rep(b, 0, j){
                if(!a && !b || a + b > p) continue;
                if(check(i - a, j - b)){
                    if(g[i - a][j - b] > f[i][j] + 1){
                        g[i - a][j - b] = f[i][j] + 1;
                        q.push({i - a, j - b, 1});
                    }
                }
            }
        }else{
            _rep(a, 0, x - i) _rep(b, 0, y - j){
                if(a + b > p) continue;
                if(check(x - i - a, y - j - b)){
                    if(f[i + a][j + b] > g[i][j] + 1){
                        f[i + a][j + b] = g[i][j] + 1;
                        q.push({i + a, j + b, 0});
                    }
                }
            }
        }
    }
    int ans = inf;
    _rep(i, 0, y) ans = min(ans, g[0][i]);
    writeln(ans == inf ? -1 : ans);
    return 0;
}