.box {
  display: flex;
  justify-content: left;
  flex-flow: row nowrap;
}

.part-1 {
  flex: 0 0 50%;
  max-width: 300px;
  padding: 10px;
}

.part-2 {
  flex: 0 0 50%;
  padding: 5px;
}

@media screen and (max-width:900px) {
  .box {
    flex-flow: column nowrap;
  }
}