uses math;
var
a, b, c: Integer;
S, p: Real;
begin
readln(a);
readln(b);
readln(c);
p := (a + b + c) / 2;
S := sqrt(p * (p - a) * (p - b) * (p - c));
writeln(S:0:1);
end.
uses math;
var
a, b, c: Integer;
S, p: Real;
begin
readln(a);
readln(b);
readln(c);
p := (a + b + c) / 2;
S := sqrt(p * (p - a) * (p - b) * (p - c));
writeln(S:0:1);
end.