#main {
  width: 400px;
  height: 400px;
  border: 1px solid #c3c3c3;
  display: flex;
  flex-direction: row-reverse;
  align-items: flex-start;
  justify-content: flex-end; <------need this
}

#main div {
  width: 50px;
  height: 50px;
}