varchar2 to float conversion

 The below procedure showing Error when column has "Double dash" --

how to handle this.. my requirement was when it was non numeric. it should print message like "Non Numeric Data" and other then this update correspondence column. 

but when found -- double dash, procedure failed.. and showing below error.  there may be other character don't know which and how to handle. 

 SQL> DECLARE

  2     my_DATA ESAM_NETCSUMRY_STATS.MAX_LATENCY%TYPE;

  3     CURSOR c1 IS SELECT MAX_LATENCY  from ESAM_NETCSUMRY_STATS for update of MAX_LATENCY_MG;

  4  BEGIN

  5     OPEN c1;

  6     LOOP

  7        FETCH c1 INTO my_DATA;

  8         EXIT WHEN c1%NOTFOUND;