To get the flag
-I opened the program in gdb
gdb debugger0_a
-put a breakpoint at main
break main
-moved using ni instruction until the needed value was put in rax
-To get the value in eax I used info registers
(gdb) ni
0x000055555555513d in main ()
(gdb) info registers
rax 0x86342 549698
rbx 0x0 0
rcx 0x555555555140 93824992235840
rdx 0x7fffffffdf58 140737488346968
rsi 0x7fffffffdf48 140737488346952