* {
  box-sizing: border-box;
}
main {
  max-width: 1200px;
  margin: 30px auto;
  padding: 0 5px;
  width: 100%; 
  display: grid;
  /* Define Auto Row size */
  grid-auto-rows: 350px; 
  /*Define our columns */
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr)); 
  grid-gap: 1em;
}

article {
  border-radius: 10px;
  height: 350px; 
  padding: 20px;
  color: #fff;
}

article:nth-child(odd) {
  background-color: #cecece;
}

article:nth-child(even) {
  background-color: #cecece;
}