#include <iostream>
using namespace std;

int main() {
    int V, T, S, D;
    cin >> V >> T >> S >> D;

    bool hit = (D < V * T || V * S < D) ? true : false;
    if (hit) cout << "Yes" << endl;
    else cout << "No" << endl;
}

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

#define REP(i, n) for(int i = 0; i < (int)(n); ++i)
#define FOR(i, a, b) for(int i = (int)(a); i <= (int)(b); ++i)
#define FORR(i, a, b) for(int i = (int)(a); i >= (int)(b); --i)
#define ALL(c) (c).begin(), (c).end()

using ll = long long;
using VI = vector<int>;
using VL = vector<ll>;
using VD = vector<double>;
using VII = vector<VI>;
using VLL = vector<VL>;
using VDD = vector<VD>;
using P = pair<int, int>;
using PL = pair<ll, ll>;

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

void solve(long long A, long long B, long long C, long long D) {
  ll div = C * D - B;
  if(div <= 0) {
    cout << -1 << endl;
    return;
  }
  cout << (A + div - 1) / div << endl;
}

int main() {
  long long A;
  std::scanf("%lld", &A);
  long long B;
  std::scanf("%lld", &B);
  long long C;
  std::scanf("%lld", &C);
  long long D;
  std::scanf("%lld", &D);
  solve(A, B, C, D);
  return 0;
}
