Changing data type from varchar to integer

Question

I'm trying to find the first free number (SYMBOL_ZAM) in a table and add 1, to make a unique one for my new record. First I'm selecting the last added number into a varchar variable (OST_SZUK_NR) and then I was trying to make field NOWY_SYMB_ZAM the same value as OST_SZUK_NR but declaring it as a integer, so I can add the 1, but it didn't work. Any tips how I can make this work?

begin
    -- POBRANIE DANYCH DO ZALOZENIA NAGLOWKA
     for select KATEGORIA, ROK_ZAM, SYMBOL_ZAM from M_ZAMWLASNE
     WHERE SYMBOL_ZAM =:symbol_zam_wyk
     into :KATEGORIA, :ROK_ZAM, :SYMBOL_ZAM
     DO begin
        -- STWORZENIE NAGLOWKA

        --OKRESLENIE DO KTOREJ BIBLIOTEKI WPADNIE OFERTA
        if (:KATEGORIA IN ('KOI','KOM','KOL')) then
            parametr='TOK';
        if (:KATEGORIA LIKE ('KOS')) then
            PARAMETR='SPA';
            --WYSZUKANIE NAJBLIZSZEGO WOLNEGO SYMBOLU ZLECENIA
            FOR select symbol_zam FROM m_zamwlasne 
            WHERE KATEGORIA=:parametr AND ROK_ZAM=:rok_zam
            ORDER BY symbol_zam desc
            INTO: OST_SZUK_NR
            --PRZEROBIENIE WYSZUKANEGO NUMERU Z VARCHAR NA INTEGER I DODANIE 1
            nowy_symb_zam=:ost_szuk_nr
            nowy_symb_zam=:nowy_symb_zam + 1