program strazi2;
type matt=array [1..256,1..256] of integer;
var A : matt;
    f : text;
    M,N,i,j,k,nr : integer;
begin
assign(f,'strazi.in');
reset(f);
readln(f,N,M);

for i := 1 to N do
for j := 1 to N do
A[i,j] := 0;

for k := 1 to M do begin
readln(f,i,j);
A[i,j] := 1;
end;
close(f);

nr := 0;
for i := 1 to n-1 do
if A[i,i+1]=0 then nr := nr+1;

assign(f,'strazi.out');
rewrite(f);

writeln(f,nr);

for i := 1 to n-1 do
if A[i,i+1] = 0 then writeln(f,i,i+1);

for i := 1 to n do
write(f,i);

close(f);
end.