const Main = (input) => {
  input = input.trim().split('\n')
  const tmp = input[0].split(' ')

  let result = ''
  while (true) {
    tmp[2] = tmp[2] - tmp[1]
    if (tmp[2] <= 0) {
      result = 'yes'
      break
    }

    tmp[0] = tmp[0] - tmp[3]
    if (tmp[0] <= 0) {
      result = 'no'
      break
    }
  }

  console.log(result)
}
Main(require('fs').readFileSync('/dev/stdin', 'utf8'))
