#include<bits/stdc++.h>
using namespace std;
int main(){
  int V,T,S,D;
  cin>>V>>T>>S>>D;
  if (D<V*T || D>V*S){
    cout<<"Yes"<<endl;
  }
  else{
    cout<<"No"<<endl;
  }
}
#define _CRT_SECURE_NO_WARNINGS
#include<bits/stdc++.h>
#include<unordered_map>
using namespace std;
clock_t T;
#define ctime cerr << "Time : " << double(clock() - T) / CLOCKS_PER_SEC << endl
#define all(v) ((v).begin()), ((v).end())
#define sz(v) v.size()
#define endl "\n"
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pii;
constexpr auto EPS = 1e-7;
const int mod = (int)1e9 + 7;
int dxn[] = { -1,-2,-2,-1,1,2,2,1 };

int dyn[] = { -2,-1,1,2,2,1,-1,-2 };
int dx[] = { -1 , -1 , -1 , 0 , 0 , 1 , 1 , 1 , 0 };
int dy[] = { -1 , 0 , 1 , -1 , 1 , -1 , 0 , 1 , 0 };
ll gcd(ll a, ll b) { return (a) ? gcd(b % a, a) : b; }
ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }
void COME_BACK()
{
    std::ios_base::sync_with_stdio(0); cin.tie(NULL); cout.tie(NULL);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    //freopen("input.txt", "w", stdout);
#endif
}

   int main()
   {
       COME_BACK();
       int a, b, c, d;
       cin >> a >> b >> c >> d;
       cout <<( a * d) - (c * b) << endl;
       
   }
