Nested IF formula with vlookup

in cell D5 =IF(VLOOKUP(C5,'store1'!$F$4:$F$29,1)=C5,"store1","")&(IF(VLOOKUP(C5,'store2'!$F$4:$F$29,1)=C5,"\store2","")&(IF(VLOOKUP(C5,'store3'!$F$4:$F$29,1)=C5,"\store3","")&(IF(VLOOKUP(C5,'store4'!$F$4:$F$29,1)=C5,"\store4","")&(IF(VLOOKUP(C5,'store5'!$F$4:$F$29,1)=C5,"\store5","")&(IF(VLOOKUP(C5,'store6'!$F$4:$F$29,1)=C5,"\store6","")&(IF(VLOOKUP(C5,'store7'!$F$4:$F$29,1)=C5,"\store7","")&(IF(VLOOKUP(C5,'store8'!$F$4:$F$29,1)=C5,"\store8","")&(IF(VLOOKUP(C5,'store9'!$F$4:$F$29,1)=C5,"\store9","")&(IF(VLOOKUP(C5,'store10'!$F$4:$F$29,1)=C5,"\store10",""))))))))))

I have ten store sheets with working employees name and one "all store employees name" sheet. Sometime few employees work in two store and swipe with store1 to store2 or store3 or other.

What I want is if some employee worked in more then one store then store(mean sheet name) number show in 'all store employees name' sheet.

Example: "all store employees name" c5 to c30 is A to Z alphabet and f4 to f29 A to Z alphabet in all store This for formula work but not popper: A to M show #n/a

Please help and thanks in advance.

I apologize for not being word-perfect in English.