#####

##def cfrac_n(f, n) :
##    val = f(n)
##    for i in range(n-1, -1, -1) :
##        val = f(i) + 1/val
##    return val
##
##def gen(n) :
##    return 1
##
##def gen1(n) :
##    if n%3 == 0 :
##        return 0
##    else:
##        return 1

## cfrac_n(gen1, 6) will cause an error

##def genS(n) :
##    return 1 if n == 0 else 2
##
##def genE(n) :
##    if n == 0 :
##        return 2
##    elif n%3 != 2 :
##        return 1
##    return (n+1)//3 * 2


#### A program with bugs for debugging
##def is_pal(x):
##    """assumes x is a 1ist 
##    returns True if x is a palindrome; False otherwise"""
##    temp = x
##    temp.reverse
##    if temp == x:
##        return True
##    else:
##        return False
##
##def judge(n):
##    """assumes n is an int and n > 0
##    Gets n inputs from user
##    Prints 'Yes' if input sequence forms a palindrome;
##    'No' otherwise"""
##    for i in range(n):
##        result = []
##        elem = input('Enter item: ')
##        result.append(elem)
##    if isPal(result):
##        print("Yes")
##    else:
##        print("No")


## Code for requiring an integer
##while True:
##    try:
##        x = int(input("Please enter an integer: "))
##        break
##    except ValueError:
##        print("Error! Input was not a valid integer. Try again ... ")


## Code for requiring name of a data file
##while True :
##    fname = input("Please give the file name: ")
##    try :
##        infile = open(fname)
##        break
##    except OSError :
##        print("Cannot open " + fname + ".", "Another...")
##
##infile.close()

##def inputInt():
##    """Function for input an integer"""
##    while True:
##        try:
##            x = int(input("Please enter an integer: "))
##            return x
##        except ValueError:
##            print("Error! Input was not a valid integer."
##                  "Try again ... ")


##def inputVal(valType, requestMsg, tpName):
##    """Generic function for input a value of a given type."""
##    while True:
##        val = input(requestMsg + " ")
##        try:
##            val = valType(val)
##            return val
##        except ValueError:
##            print(val + " is not of type " + tpName + ".",
##                  "Try again...")


##def getDataFileName ():
##    while True :
##        fname = input("Please give the file name: ")
##        try :
##            infile = open(fname)
##            return infile
##        except OSError :
##            print("Cannot open " + fname + ".", "Another...")


from math import sqrt
def triangle(a, b, c) :
    if a > 0 and b > 0 and c > 0 \
       and a+b > c and a+c > b and b+c > a :
        s = (a + b + c) / 2
        return sqrt(s * (s - a) * (s - b) * (s - c))
    else:
        raise ValueError("wrong argument(s) for function triangle")


def testTri () :
    try :
        print(triangle(3, 5, 6))
        print(triangle(2, 4, 7))
    except ValueError :
        print("Find error.")



def testTri1 () :
    try :
        print(triangle(3, 5, 6))
        print(triangle(2, 4, 7))
    except ValueError as msg :
        print("Find error:", msg)


def triangle(a, b, c) :
    if a > 0 and b > 0 and c > 0 \
       and a+b > c and a+c > b and b+c > a :
        s = (a + b + c) / 2
        return sqrt(s * (s - a) * (s - b) * (s - c))
    else:
        raise ValueError("wrong argument(s) for function triangle",
                         (a, b, c))