#grid-container {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 5px;
}
#grid-container div {
  padding: 20px;
  background: #CCCCCC;
}
#box-1 {
  grid-column-start: 1;
  grid-column-end: 4;
}
#box-2 {
  grid-column-start: 1;
  grid-column-end: 3;
  grid-row-start: 2;
  grid-row-end: 4;
}