#include <stdio.h>
#include <string.h>
#include<stdlib.h>

int main()
{
    int a, b ,c, d;
    scanf("%d %d %d %d", &a, &b, &c, &d);
    int m, l;
    if(c%b==0){
        m = c/b;
    }else{
        m = c/b+1;
    }
    if(a%d==0){
        l = a/d;
    }else{
        l = a/d+1;
    }
    if(m>l){
        printf("no");
    }else{
        printf("yes");
    }
}
