PROGRAMMING:Triangle area (Helen formula)

Input the length a, B, C of the three sides of the triangle, and output its area s (assuming that the user's input a, B, C can form a triangle).
Given the length of three sides of a triangle, we can use Helen's formula to calculate its area.
Let the three sides of triangle be a, B and C respectively, $$p = {(a + B + C)} / {2} $$,
Then the Helen formula for calculating the area of the triangle is: $$s = [sqrt {p * (P-A) * (P-B) * (P-C)} $$.
###Input example:
```in
3 4 5
```
###Output example:
```out
six
```
###Input example:
```in
6 8 10
```
###Output example:
```out
twenty-four
```







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