http://codeforces.net/contest/767/problem/C

I have done some very basic questions of DFS , but I always feel confused and not so sure about , how to systematically formulate my solution in the dfs code . I feel confused , wheather to place certain statements before or after the recurssive call. A similar situation happens for this problem too . I came up with the idea , but feel lost when I have to code it in the dfs solution. I would request this community , to please share your approaches towards handling such DFS codes . Thanks :)