#pragma GCC optimize ("Ofast")
#define _USE_MATH_DEFINES
//#include <bits/stdc++.h>
# include <iostream>
# include <array>
# include <string>
# include <vector>
# include <algorithm>
# include <functional>
# include <cmath>
# include <iomanip>
# include <stack>
# include <queue>
# include <numeric>
# include <map>
# include <set>
# include <unordered_map>
# include <unordered_set>
# include <chrono>
# include <random>
# include <bitset>
# include <cassert>
# include <climits>

template<typename T> inline bool chmax(T &a, T b) { return ((a < b) ? (a = b, true) : (false)); }
template<typename T> inline bool chmin(T &a, T b) { return ((a > b) ? (a = b, true) : (false)); }

#define rep2(i,t,n) for (int i = (t); i < (int)(n); i++)
#define rep(i,n) rep2(i,0,n)
#define YESNO(T) if(T){cout<<"YES"<<endl;}else{cout<<"NO"<<endl;}
#define yesno(T) if(T){cout<<"yes"<<endl;}else{cout<<"no"<<endl;}
#define YesNo(T) if(T){cout<<"Yes"<<endl;}else{cout<<"No"<<endl;}

using namespace std;
typedef long long ll;

ll a,b,c,d;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  cin >> a >> b >> c >> d;

  if(d*c <= b) {
    cout << -1 << endl;
    return 0;
  }


  ll aka = 0;

  ll count = 0;

  while(a > d*aka) {
    a += b;
    aka += c;
    count++;
  }

  cout << count << endl;

  return 0;
}

#include <bits/stdc++.h>
#define ll int64_t
using namespace std;

int dfs(vector<vector<bool>>& used, int a, int b, int r, int c) {
	int h = used.size();
	int w = used[0].size();

	if(r == h) return 1;
	int r2, c2;
	if(c == w-1) r2=r+1, c2=0;
	else r2=r, c2=c+1;

	assert(0<=c && c<w && 0<=r && r<h);

	if(used[r][c]) return dfs(used, a, b, r2, c2);
	int ans = 0;
	if(b > 0) {
		used[r][c] = true;
		ans += dfs(used, a, b-1, r2, c2);
		used[r][c] = false;
	}
	if(a > 0 && c < w-1) {
		if (!used[r][c + 1]){
      used[r][c] = true;
      used[r][c + 1] = true;
      ans += dfs(used, a-1, b, r2, c2);
      used[r][c] = false;
      used[r][c + 1] = false;
    }
  }
  if (a > 0 && r < h-1){
    if (!used[r + 1][c]){
      used[r][c] = true;
      used[r + 1][c] = true;
      ans += dfs(used, a-1, b, r2, c2);
      used[r][c] = false;
      used[r + 1][c] = false;
    }
  }
  return ans;
}

void solve() {
	int h, w, a, b;
	cin >> h >> w >> a >> b;
	
	vector<vector<bool>> used(h, vector<bool>(w, false));
	cout << dfs(used, a,b,0,0) << '\n';
}

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	
	int t = 1;
	// cin >> t;
	while(t--) {
		solve();
	}
	
	return 0;
}