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.