Build a simple command-line calculator.


The program should:
1. Read a mathematical expression from stdin
2. Evaluate the expression
3. Print the result to stdout

Support these operations:
- Addition (+)
- Subtraction (-)
- Multiplication (*)
- Division (/)

Examples:
Input: "2 + 2"
Output: "4"

Input: "10 * 5"
Output: "50"

The program should handle integer arithmetic. You may use any programming language.

The root of the project just contain `./run` script that that would build and run the application. For example a caller would: `echo "1+2" | ./run` and get "3" in the stdout.
