/in/foo.cc: In function 'void dfs(int)':
/in/foo.cc:9:6: warning: unused variable 'opt' [-Wunused-variable]
  int opt=inf;
      ^~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:27:2: error: 'else' without a previous 'if'
  else dfs(s);
  ^~~~