#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> ii;
typedef vector<int> vi;
typedef vector<ii> vii;
#define FOR(i, a, b) for(int i = a; i < b; i++)
#define ROF(i, a, b) for(int i = a; i >= b; i--)
#define ms memset
#define pb push_back
#define F first
#define S second
ll MOD = 998244353;
 
ll power(ll base, ll n){
	if (n == 0) return 1;
	if (n == 1) return base;
	ll halfn = power(base, n/2);
	if (n % 2 == 0) return (halfn * halfn) % MOD;
	return (((halfn * halfn) % MOD) * base) % MOD;
}
 
ll inverse(ll n){
	return power(n, MOD-2);
}

ll add(ll a, ll b){
	return (a+b) % MOD;
}

ll mul(ll a, ll b){
	return (a*b) % MOD;
}

ll gcd(ll a, ll b){
	if (a == 0) return b;
	if (a == 1) return 1;
	return gcd(b%a, a);
}

int main(){
	ios::sync_with_stdio(false);
	int v, t, s, d; cin >> v >> t >> s >> d;
	if (v * t > d || v * s < d){
		cout << "Yes\n";
	}else{
		cout << "No\n";
	}
	return 0;
}




/*
    ->->->->-> Vora Mahammasasim <-<-<-<-<-
*/
#include<bits/stdc++.h>
#define Quick std :: ios_base :: sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define int long long
#define pb emplace_back
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define MOD 1000000007
using namespace std;


void solve(){

    int a,b,c,d;
    cin>>a>>b>>c>>d;
    cout<<a*d-b*c<<'\n';

}
signed main(){

    Quick;

    int tt = 1;
    //cin>>tt;
    while(tt--){
        solve();
    }
    return 0;
}
