area of a triangle

#!/usr/bin/python3
from math import sqrt
a = float(input('a = '))
b = float(input('b = '))
c = float(input('c = '))
if a < b + c and b < c + a and c < a + b:
    s = (a + b + c) / 2
    print(f'S = {sqrt(s * (s - a) * (s - b) * (s - c))}')
else:
    print(-1)