/in/foo.cc: In function 'int main()':
/in/foo.cc:11:15: error: too few arguments to function 'int dfs(int, int)'
  int ans=dfs(n);
               ^
/in/foo.cc:4:5: note: declared here
 int dfs(int last,int top){
     ^~~
/in/foo.cc: In function 'int dfs(int, int)':
/in/foo.cc:7:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^