var head,vet,next,d,x,y,b:array[..]of longint;

     n,m,i,l,r,mid,last,tot:longint;

 procedure add(a,b:longint);

 begin

  inc(tot);

  next[tot]:=head[a];

  vet[tot]:=b;

  head[a]:=tot;

 end;

 function isok(len:longint):boolean;

 var s,e,i,st,v:longint;

 begin

  fillchar(head,sizeof(head),);

  fillchar(b,sizeof(b),);

  fillchar(d,sizeof(d),);

  tot:=;

  for i:= to len do

  begin

   add(x[i],y[i]);

   inc(d[y[i]]);

  end;

  s:=;

  for i:= to n do

   if d[i]= then begin st:=i; b[i]:=; inc(s); end;

  if s<> then exit(false);

  for i:= to n- do

  begin

   e:=head[st];

   while e<> do

   begin

    v:=vet[e];

    dec(d[v]);

    e:=next[e];

   end;

   e:=head[st]; s:=;

   while e<> do

   begin

    v:=vet[e];

    if (d[v]=)and(b[v]=) then

    begin

     inc(s); b[v]:=; st:=v;

    end;

    e:=next[e];

   end;

   if s<> then exit(false);

  end;

  exit(true);

 end;

 begin

 // assign(input,'cf645D.in'); reset(input);

 // assign(output,'cf645D.out'); rewrite(output);

  readln(n,m);

  for i:= to m do readln(x[i],y[i]);

  l:=; r:=m; last:=m+;

  while l<=r do

  begin

   mid:=(l+r)>>;

   if isok(mid) then begin last:=mid; r:=mid-; end

    else l:=mid+;

  end;

  if last=m+ then writeln(-)

   else writeln(last);

  //close(input);

 // close(output);

 end.