Please, give me some hint!

Standard Prim's(Greedy) algo works if we assign -d to edges
in graph.

nothing special, just obtain a spanning tree (forest)... I think you made a confusion between verteces and parents, or something like that... try this example:
7
2 10
0000101
0000101
0001010
0010010
1100001
0011000
1100100

One of the correct answers is
16
00a000d
0000000
a0000d0
0000000
000000d
00d0000
d000d00

I've WA6 too. But on pervious test I've got AC. I use connected components of graph and Kruskal algo for builting spanning tree.

Try to estimate the maximal possible answer to this problem.

Thanks you. This test helped me to find the reason of WA6.

Had a very silly WA6, and none of tests from this or neighbouring topics helped, so here's the one that i finally thought of and which helped:
5
1 1
01000
10001
00010
00101
01010
(1-2, 2-5, 5-4, 4-3, result is all zeros.)
I've been sleepy and lazy and did a bad job with a tree building algo, which i made like this:
---
(mark vertices 1..N as unconnected)
for i:=1 to N do
    for j:=1 to N do
        if (i, j) is an existing edge and either i or j is yet unconnected to anything,
            then add (i, j) to a tree and mark both i and j as connected.
---
So then it adds 1-2, then 2-5, then 3-4, and then it appears that both 4 and 5 are already connected to something, and 4-5 isn't included to a tree, which is a mistake.

My programm have wa6? and on your test give:
16
0000000
000000d
000000a
00000d0
000000d
000d000
0da0d00

Is it correct?

helpful!thx