1 var i,n,m:longint;
 2     p:array[0..1500] of longint;
 3     v:array[0..1500] of boolean;
 4     a:array[0..1500,1..2] of longint;
 5 function find(x:longint):boolean;
 6  var i,y:longint;
 7  begin
 8  for i:=1 to 2 do
 9   begin
10   y:=a[x,i];
11   if v[y] then continue;
12   v[y]:=true;
13   if (p[y]=0) or (find(p[y])) then
14    begin
15    p[y]:=x;
16    exit(true);
17    end;
18   end;
19  exit(false);
20  end;
21 procedure main;
22  begin
23  readln(n,m);
24  for i:=1 to m do begin readln(a[i,1],a[i,2]);inc(a[i,1]);inc(a[i,2]);end;
25  for i:=1 to m do
26   begin
27   fillchar(v,sizeof(v),false);
28   if not(find(i)) then begin writeln(i-1);halt;end;
29   end;
30  writeln(m);
31  end;
32 begin
33  main;
34 end.