Hi, here's your problem today. This problem was recently asked by Google:

Given a mathematical expression with just single digits, plus signs, negative signs, and brackets, evaluate the expression. Assume the expression is properly formed.

Example:

```
Input: - ( 3 + ( 2 - 1 ) )
Output: -4
```

Here's the function signature:

```
def eval(expression):
# Fill this in.
print eval('- (3 + ( 2 - 1 ) )')
# -4
```