I have to add number in column 1 by month.

1531 Feb 1 2007 pgaps_p007_27909.trc
1529 Feb 1 2007 pgaps_p006_27907.trc
1533 Feb 1 2007 pgaps_p005_27905.trc
635 Apr 7 2007 pgaps_smon_2347.trc
634 Apr 7 2007 pgaps_lgwr_2343.trc
634 Apr 8 2007 pgaps_lgwr_2311.trc
613 Apr 10 2007 pgaps_p004_402.trc
615 Apr 10 2007 pgaps_p002_7500.trc
698 May 2 2007 pgaps_smon_2339.trc
634 May 5 2007 pgaps_lgwr_2335.trc
656 Mar 1 2007 pgaps_smon_8006.trc
634 Mar 3 2007 pgaps_lgwr_8002.trc
634 Mar 4 2007 pgaps_lgwr_2383.trc
641 Mar 8 2007 pgaps_qmn0_20351.trc

Output should be :
Feb 4593
Apr 3131
May 3897

When I run below awk command I am not getting desired output.

ls -ltr |awk 'BEGIN { OFS = "\t"; ORS = "\n\n" } { print $1, $2, $3, $4 }'| awk '{tot=tot+$1}END{print tot}'

Any help would be greatly appreciated.