View Compilation Error

0_0_35365163_15490.cpp: In function 'void dfs(int)':
0_0_35365163_15490.cpp:32:26: error: unable to find numeric literal operator 'operator""n'
  for(int i=0;i<num&&a[i]<2n-1;i++){
                          ^
0_0_35365163_15490.cpp:32:26: note: use -std=gnu++11 or -fext-numeric-literals to enable more built-in suffixes