## Explanation

Working from the inside out, we first "normalize" row numbers to begin with 1 using the ROW function and an offset:

```
ROW()-offset
```

In this case, the first row of data is in row 5, so we use an offset of 4:

```
ROW()-4 // 1 in row 5
ROW()-4 // 2 in row 6
ROW()-4 // 3 in row 7
etc.
```

The result goes into the CEILING function, which rounds incoming values up to a given multiple of n. Essentially, the CEILING function counts by a given multiple of n:

This count is then divided by n to count by groups of n, starting with 1:

Finally, the ISEVEN function is used to force a TRUE result for all even row groups, which triggers the conditional formatting.

Odd row groups return FALSE so no conditional formatting is applied.

### Shade first group

To shade rows starting with the the first group of n rows, instead of the second, replace ISEVEN with ISODD:

```
=ISODD(CEILING(ROW()-offset,n)/n)
```