# Solution

Just add `serialize_word(400);`

# Explanation

> The function serialize_word(): To write the value x to the output, we can use the library function serialize_word(x). serialize_word gets one argument (the value we want to write) and one implicit argument output_ptr (which means that behind the scenes it also returns one value). In fact it’s quite simple: it writes x to the memory cell pointed by output_ptr (that is, [output_ptr]) and returns output_ptr + 1. Now the implicit argument mechanism kicks in: in the first call to serialize_word() the Cairo compiler passes the value of output_ptr as the implicit argument. In the second call it uses the value returned by the first call.

**Source:** [https://www.cairo-lang.org/docs/hello_cairo/intro.html](https://www.cairo-lang.org/docs/hello_cairo/intro.html)

# Code

```
// 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);
// Output 400.
serialize_word(400); // << Solution
// Return.
return ();
}
```

# Program Output

```
Program output:
  100
  200
  300
  400

Number of steps: 22
Program hash: 0x0542e6604d143454ef2a63b5e6762616c9999e2224bc7d6098a1b9617fa9dd72
Program output:
  100
  200
  300
  400

Number of steps: 22
Program hash: 0x0542e6604d143454ef2a63b5e6762616c9999e2224bc7d6098a1b9617fa9dd72
```

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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