PROGRAMMING:JMU Python combinatorial number

The problem requires the preparation of procedures, according to the formula

Calculate the combination number of M elements (m ≤ n) from n different elements.
###Input format:
Enter two positive integers m and N in one line, separated by spaces.
-If the input data is negative, it can output a prompt that it cannot be negative`
-If the input data starts from illegal data, the prompt "please input value" can be output (value exception handling)
###Output format:
The result is output according to the format 'result = combination number'.
###Input example:
```in
2 7
```
###Output example:
```out
result=21.00
```







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