{
ZADATAK: Meteor
JEZIK: PASCAL
}
program prvi;
type point=record
             x, y: double;
           end;

var x, y, r: double;
    n: int64;
    p: array [0..7] of point;
    maxx, minx, maxy, miny: longint;

procedure load;
begin
  readln(x, y, r);
end;

procedure solve;
var i, j: longint;
begin
  maxx:=trunc(x+r);
  minx:=trunc(x-r);if minx>=0 then inc(minx);
  maxy:=trunc(y+r);
  miny:=trunc(y-r);if miny>=0 then inc(miny);

  n:=n+(maxx-minx)*(maxy-miny);
  if maxx<x+r then n:=n+maxy-miny-2;
  if minx>x-r then n:=n+maxy-miny-2;
  if maxy<y+r then n:=n+maxx-minx-2;
  if miny>y-r then n:=n+maxx-minx-2;

end;


begin{program}

  load;
  solve;
  write(n);
end.