#include <bits/stdc++.h>

using namespace std;

#define pb push_back
#define pii pair<int, int>

const int maxN = 1e6 + 10;

int n;
int a[maxN];

int main()
{  
	long long  v, t, s, d;

	cin>>v>>t>>s>>d;

	long long l = v*t;
	long long r  = v*s;

	if (d>r || d<l) printf("Yes\n"); else printf("No\n"); 
	return 0;
}
#include <bits/stdc++.h>
using namespace std;

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