How do I reformat a selection of cells in order for my values to be selected and dragged down creating cumulative numbering

eg if i select 1-01 and 1-02 and drag, i want it to create 1-03, 1-04 etc

my current settings re-create 1-01, 1-02, 1-01, 1-02, 1-01, 1-02 etc

the cells dont appear to be effected by formatting

any ideas?