function ValidateAccount(const acc, pzn, bik:string):boolean;

        function CalcAccKey(const Account, BIK, PZN:string; const WithKey:boolean):integer;

                  function CalcKey(const Acc, Mask, Koef:string):integer;
                  var i:integer;
                  begin
                    result:= 0 ;
                    for i:= 1  to length(Mask) do
                      result:=result+(StrToInt(Acc[i])*StrToInt(Mask[i])*StrToInt(Koef[i]));
                  end;
        const
          AccMask1='11111111011111111111';
          AccMask2='11111111111111111111';
          AccKoef ='71371371371371371371';
          BikMask1='000000111';
          BikKoef1='000000713';
          BikMask2='000011000';
          BikKoef2='000013000';

        var AccMask, BikMask, BikKoef, sum:string;
        begin
          if length(PZN)<> 2  then raise Exception.Create('PZN пустой. Возможно, не указан банк');
          if WithKey then AccMask:=AccMask2 else AccMask:=AccMask1;
          if (PZN<>'00') and (PZN<>'10') then
          begin
            BikMask:=BikMask1;
            BikKoef:=BikKoef1;
          end
          else
          begin
            BikMask:=BikMask2;
            BikKoef:=BikKoef2;
          end;
          result:=CalcKey(Account, AccMask, AccKoef)+CalcKey(BIK, BikMask, BikKoef);
          sum:=IntToStr(result);
          result:=StrToInt(sum[length(sum)])* 3 ;
        end;

begin
  if CalcAccKey(acc, bik, pzn, true)<> 0  then
  begin
    Application.MessageBox(PChar('Счет не ключуется'), PChar(Application.Title), MB_OK + MB_ICONSTOP);
    result:=false;
    exit;
  end;
  result:=true;
end;