# Problem Definition

## Description

Write a function that takes two strings and returns a string containing letters present in both inputs.

**Examples:**

```plaintext
f("abcd", "efgh") => ""
f("aeiou", "abcde") => "ae"
f("aeiou", "abcabc") => "a"
f("Call me Ishmael…", "It was best of times, it was the worst of times...")
f("Call", "It was the best of times…")
f("Call me Ishmael…", "abcdef…xyz")
```

## Notes