I would like to know the meaning of "--" (double dash) in the below Excel formula:

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=SUM(OFFSET(Source!$C$4:$C$425,,,MATCH(0,--(MMULT(--(ROW(OFFSET($A$1,,,ROWS(Source!$C$4:$C$425)))=COL UMN(OFFSET($A$1,,,,ROWS(Source!$C$4:$C$425))))-0.5*(ROW(OFFSET($A$1,,,ROWS(Source!$C$4:$C$425)))= COLUMN(OFFSET($A$1,,,,ROWS(Source!$C$4:$C$425)))), OFFSET(Source!$C$4:$C$425,,,ROWS(Source!$C$4:$C$42 5))+0)SUM(Source!$C$4:$C$425)*ROWS($C$4:$C5)/4))))-SUM(OFFSET(Source!$C$4:$C$425,,,MATCH(0,--(MMULT(--(ROW(OFFSET($A$1,,,ROWS(Source!$C$4:$C$425)))=COL UMN(OFFSET($A$1,,,,ROWS(Source!$C$4:$C$425))))-0.5*(ROW(OFFSET($A$1,,,ROWS(Source!$C$4:$C$425)))= COLUMN(OFFSET($A$1,,,,ROWS(Source!$C$4:$C$425)))), OFFSET(Source!$C$4:$C$425,,,ROWS(Source!$C$4:$C$42 5))+0)SUM(Source!$C$4:$C$425)*ROWS($C$4:$C4)/4))))
========= end of formula =================

Thanks.