program sd;
uses crt;
var
n, a, t: integer;
function sum(a, n, t: integer): integer;
begin
if t > 1 then
result := result + sum(a, n, t - 1) + n
else if (t = 1) then
result := a;
end;
begin
read(a);
read(n);
read(t);
write(sum(a, n, t));
end.