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

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
#define FOR(i, n) for(int (i)=0; (i)<(n); (i)++)
#define FOR1(i, n) for(int (i)=1; (i)<=(n); (i)++)
#define FORI(i, n) for(int (i)=n-1; (i)>=0; (i)--)
template<class T, class U> void umin(T& x, const U& y){ x = min(x, (T)y);}
template<class T, class U> void umax(T& x, const U& y){ x = max(x, (T)y);}
template<class T> void print(T& v){ for(auto it : v) cout << it << " "; cout << endl; }
template<class T, class U> void init(vector<T> &v, U x, size_t n) { v=vector<T>(n, (T)x); }
template<class T, class U, typename... W> void init(vector<T> &v, U x, size_t n, W... m) { v=vector<T>(n);  for(auto& a : v) init(a, x, m...); }

int main(int argc, char** argv) {
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0); cout << setprecision(15);
    if (argc == 2 && atoi(argv[1]) == 123456789) freopen("d:\\code\\cpp\\contests\\stdin", "r", stdin);

    int x, y;
    cin >> x >> y;
    if (x == y)
        cout << x << endl;
    else
        cout << 3-x-y << endl;

    if (argc == 2 && atoi(argv[1]) == 123456789) cout << clock()*1.0/CLOCKS_PER_SEC << " sec\n";
    return 0;
}


/*
 * @Author: tusikalanse
 * @Date: 2021-06-26 19:48:54
 * @LastEditTime: 2021-06-26 20:04:29
 * @LastEditors: tusikalanse
 * @Description: 
 * @FilePath: /atcoder/abc/207/b.cpp
 */
#include <bits/stdc++.h>
using namespace std;
using LL = long long;
LL gcd(LL a, LL b) {
    return b == 0 ? a : gcd(b, a % b);
}
const int INF = 0x3f3f3f3f;
const LL INFL = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
//-------------------end head--------------


int main() {
    long long a, b, c, d;
    cin >> a >> b >> c >> d;
    if (b >= d * c) {
        cout << -1 << endl;
        return 0;
    }
    cout << 1 + (a - 1) / (d * c - b) << endl;

    return 0;
}
