Var a,b,R:LongInt; Shcn,Sht,Str:Real;
      fi,fo:text;
Begin
     Assign(fi,'dttr.INP'); Reset(fi);
    Assign(fo,'dttr.OUT'); Rewrite(fo);
    Read(fi,a,b,R);
    Shcn:=a*b;
    Sht:=r*r*3.14;
    Str:= Shcn-Sht;
    Write(fo,Str:0:2);
    Close(f); close(g);
end.