int2+float8 problems

select trn_qty,amount2,trn_qty*amount2 from invo

trn_qty INT2
amount2 FLOAT8

+---------+---------+----------+
| trn_qty | amount2 | ?column? |
+---------+---------+----------+
|      -1 |    7678 |     7678 |

what to get -7678

thanks