#include <stdio.h>
#include <math.h>
int main(void)
{
    int A, B, C, D;
    scanf("%i %i %i %i", &A, &B, &C, &D);
    while (1)
    {
        C -= B;
        if (C <= 0)
        {
            printf("yes\n");
            return 0;
        }
        A -= D;
        if (A <= 0)
        {
            printf("no\n");
            return 0;
        }
    }
}