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

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

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<ll> vi;
typedef pair<ll,ll> pi;
typedef vector<pi> vp;
typedef set<ll> si;
typedef map<string, ll> msi;
#define REP(i,a,b) \
for(ll i=ll(a);i<=ll(b);i++)
#define DEP(i,a,b) \
for(ll i=ll(a);i>=ll(b);i--)
#define TR(c, it) \
for (auto it = (c).begin(); it != (c).end(); it++)
#define mp make_pair
#define pb push_back
#define F first
#define S second
ll mod = 1e9+7;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  
  int T;
  T=1;
  while(T--)
  {
      ll n,i,j,k,l,m,v,t,s,d;
      cin>>v>>t>>s>>d;
      if(v*t<=d&&s*v>=d)cout<<"No";
      else cout<<"Yes";
  }
  return 0;
}