//بسم الله الرحمن الرحيم

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef pair<int,int> pii;
typedef pair<double,double> pdd;
typedef pair<ll,ll> pll;
typedef vector<pii> vii;
typedef vector<pll> vll;
//using u128 = __uint128_t;
#define el << '\n'
#define sz(v) ((int)(v).size())
#define PI 3.141592653589793238
#define _test_ int _ ; cin >> _ ; while( _-- )
#define optimize() ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define file() freopen("input.txt", "r", stdin); freopen("output.txt", "w", stdout)
#define pb push_back
ll gcd ( ll  a, ll b ) { return __gcd ( a, b );}
ll lcm ( ll a, ll b ) { return a * ( b / gcd ( a, b ) );}
ll big_mod(ll base , ll power , ll mod){
    ll ret=1;base%=mod;while(power){if(power&1)ret=ret*base%mod;base=base*base%mod;power>>=1;}return ret;}
ll mod_inverse(ll a , ll mod ){ return big_mod(a,mod-2,mod);}
ull POW(ll a , ll b){if(b==0) return 1;ull ret=1;while(b--)ret*=a;return ret;}
int dx[]={0,0,+1,-1,+1,-1,-1,+1};
int dy[]={+1,-1,0,0,+1,-1,+1,-1};
// ------------------>
const ll infLL = 15e17+123;
const int inf = 1e9+7;
const ll mod = 1e9+7;
const ll mx = 1e5+123;
// ------------------>

void _case_()
{
    int v,t,s,d;
    cin >> v >> t >> s >> d;
    int a=v*t ;
    int b=v*s;
    if(d<a or d>b) cout << "Yes\n";
    else cout << "No\n";
}

int main()
{
    optimize();
    //_test_
        _case_();
    return 0;//                                                          AUTHOR : NAYEMUR RAHMAN NAYEM
}//                                                                      [ DEPT. of CSE ; CU ; ( 19 th Batch ) ]

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

int main() {
  int V,T,S,D;
  cin>>V>>T>>S>>D;
  
  if(V*T<=D && D<=V*S){
    cout<<"No";
  }
  else{
    cout<<"Yes";
  }
  
}

