gcc permission denied in opening a flex file

I installed flex program in my computer . Codeblocks were installed previously . When I try to open my flex program in cmd by typing flex l1a.l gcc lex.yy.c -o app I found the following error : program files(x86)/codeblocks/mingw/bin/../lib/gcc/mingw32/4.7.1/../../../../mingw32/bin/ld.exe: cannot open output file app.exe: Permission denied collect2.exe: error: ld returned 1 exit status how can I solve the problem ?