The function `serialize_word` gets one argument (`x` the value we want to write) and one implicit argument `output_ptr` (which means that behind the scenes it also returns one value). 

It will write `x` to the memory cell pointed by `output_ptr` (that is, `[output_ptr]`) and returns `output_ptr + 1`. 

To outputs 400, you only need to add the following call before return:
```
serialize_word(400);
```


Can you help how to change the code so that it outputs 400 as well?

```
// The following code outputs the numbers 100, 200, 300.
// 1. Click on "Run" to see the output.
// 2. Change it to output 400 as well.

// Use the output builtin.
%builtins output

// Import the serialize_word() function.
from starkware.cairo.common.serialize import serialize_word

func main{output_ptr: felt*}() {
// Output 100 by calling serialize_word.
serialize_word(100);
// Output 200.
serialize_word(200);
// Output 300.
serialize_word(300);
// Return.
return ();
}
```

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