#include <stdio.h>

int main(void) {
  int A;
  int B;
  int C;
  int D;
  scanf("%d,%d,%d,%d", &A, &B, &C, &D);
  while (1) {
    C = C - B;
    if (C <= 0) {
      printf("Yes\n");
      return 0;
    }
    A = A - D;
    if (A <= 0) {
      printf("No\n");
      return 0;
    }
  }
  return 0;
}