#include<stdio.h>
int main()
{
    long long int a,b,c,d;
    scanf("%lld%lld%lld%lld",&a,&b,&c,&d);
    while(a>=0 && c>=0)
    {
        c = c - b;
        if(c<=0)
        {
            printf("yes");break;
        }
        a = a - d;
        if(a<=0)
        {
            printf("no");break;
        }

    }
    return 0;
}