#include <bits/stdc++.h>
#define pii pair<int, int>
using namespace std;

namespace IO {
	#if ONLINE_JUDGE
	#define getc() (IS == IT && (IT = (IS = ibuf) + fread(ibuf, 1, IL, stdin), IS == IT) ? EOF : *IS++)
	#else
	#define getc() getchar()
	#endif
	const int IL = 1 << 21, OL = 1 << 21;
	int olen = 0;
	char ibuf[IL], *IS = ibuf, *IT = ibuf, obuf[OL];
	inline int read() {
		register char ch = getc(); register int x = 0, f = 1;
		while(!isdigit(ch)) { if(ch == '-') f = -1; ch = getc(); }
		while(isdigit(ch)) x = x * 10 + ch - 48, ch = getc();
		return x * f;
	}
	inline double readdb() {
		register char ch = getc(); register double x = 0, f = 1;
		while(!isdigit(ch)) { if(ch == '-') f = -1; ch = getc(); }
		while(isdigit(ch)) x = x * 10 + ch - 48, ch = getc();
		if(ch == '.') {
			register double b = 0.1;
			ch = getc();
			while(isdigit(ch)) x += (ch - 48) * b, b *= 0.1, ch = getc();
		}
		return x * f;
	}
	inline int readstr(char *s) {
		register char ch = getc(); register int len = 0;
		while(!isalpha(ch)) ch = getc();
		while(isalpha(ch)) s[++len] = ch, ch = getc();
		return len;
	}
	inline void flush() { fwrite(obuf, 1, olen, stdout); olen = 0; }
	inline void putc(register char ch) { obuf[olen++] = ch; }
	template<class T>
	inline void write(register T x) {
		if(x < 0) obuf[olen++] = '-', x = -x;
		if(x > 9) write(x / 10);
		obuf[olen++] = x % 10 + 48;
	}
} using namespace IO;
const int N = 1e2 + 10;
int x, y, p, q;
struct node {
	int a, b, op;
};
int dis[N][N][2];
bool check(int a, int b, int op) {
	if(!op) {
		if(y - b > x - a + q && x - a) return 0;
		return 1;
	}
	if(op) {
		if(b > a + q && a) return 0;
		return 1;
	}
}
queue<node> qr;
void bfs() {
	memset(dis, -1, sizeof(dis)); 
	qr.push({x, y, 0});
	dis[x][y][0] = 0;
	while(!qr.empty()) {
		int u = qr.front().a, v = qr.front().b, op = qr.front().op;
		qr.pop();
		if(!op) {
			for(int i = 0; i <= u; i++)
				for(int j = 0; j <= v && i + j <= p; j++) {
					if(check(u - i, v - j, 1) && !~dis[u - i][v - j][1]) {
						dis[u - i][v - j][1] = dis[u][v][0] + 1;
//						cout << "pre:" <<u - i << " " << v - j << " " << u << " " << v << endl;
						qr.push({u - i, v - j, 1}); 
					}
				}
				
			}
		else {
			for(int i = u; i <= x; i++)
				for(int j = v; j <= y; j++) {
					if(i - u + j - v > p) continue;
					if(check(i, j, 0) && !~dis[i][j][0]) {
						dis[i][j][0] = dis[u][v][1] + 1;
//						cout << "pre:" <<u - i << " " << v - j << " " << u << " " << v << endl;
						qr.push({i, j, 0}); 
					}
				}
			
		}
	}
}
int main() {
	x = read(), y = read(), p = read(), q = read();
	bfs();
	int ans = 2e9;
//	for(int i = 0; i <= x; i++)
//		for(int j = 0; j <= y; j++)
//			cout << i << " " << j << " " <<dis[i][j][0] << " " <<dis[i][j][1] << endl;
	for(int i = 0; i <= y; i++)
		if(~dis[0][i][1]) ans = min(ans, dis[0][i][1]); 
	if(ans < 2e9) printf("%d\n", ans);
	else puts("-1"); 
	return 0;
}