Please help me, what is wrong with my solution....
Const
 MaxN = 100000;
Var
 b : array[1..MaxN] of boolean;
 c : array[1..2,1..MaxN] of longint;
 c_num,n,m,i,x,y : longint;
BEGIN
 Readln(n,m);
 Fillchar(b,sizeof(b),true);
 Fillchar(c,sizeof(c),0);
 c_num := 0;
 For i := 1 to m do
  begin
   Read(x,y);
   if b[x] and b[y] then
    Begin
     Inc(c_num);
     c[1][c_num] := x;
     c[2][c_num] := y;
     b[x] := false;
     b[y] := false;
    End;
  End;
 Writeln(c_num);
 For i := 1 to C_num do
  Begin
   Writeln(c[1][i],' ',c[2][i]);
  End;
END.

Try such test... :)
4 3
2 3
1 2
3 4
Correct answer is
2
1 2
3 4

I don't think, that greedy is a good idea...

Try this one
4 3
1 2
1 3
3 4

Correct Answer:
2
1 2
3 4