The aim of this challenge is to implement a simple integer calculator. The calculator takes a string of space-separated numbers and operators (like "1 + 2 + 3" or "10 + 4 - 5") and returns the result of the calculation.
It should resolve the following sums:
- "1"
- "1 + 1"
- "1 + 2 + 3"
- "10 - 6"
- "10 + 5 - 6"
- "10 * 5"
If an operator isn't known, an exception should be thrown.