body {
  margin: 40px;
}

    .wrapper {
        display: grid;
    	grid-gap: 10px;
        grid-template-columns: repeat(auto-fit, minmax(300px, 1fr )) ;
        grid-template-rows: repeat(2, [row] auto  );
        background-color: #fff;
        color: #444;
    }

    .box {
        background-color: #444;
        color: #fff;
        border-radius: 5px;
        padding: 20px;
        font-size: 150%;

    }

    .box .box {
        background-color: #ccc;
        color: #444;
    }

    .a {
        grid-column: 0.5;
        grid-row: row;
    }

    .b {
        grid-column: 0.5;
        grid-row: row;
    }