var i,j,n,m,x,y,k:longint;
       u:byte;
       a:array[1..1000,1..1000] of byte;

begin
assign(input,'dfs.in');
assign(output,'dfs.out');
reset(input);
rewrite(output);
     read(n); readln(m);
      for i:=1 to m do
       begin
        read(x); readln(y);
         a[x][y]:=1;
       end;
       k:=n;

       for i:=1 to n do
        begin
         u:=1;
          for j:=i to n do
           begin
            if u=1 then
                       if a[i][j]=1 then u:=0;
            if j=n then if u=1 then dec(k);

           end;
        end;
        writeln(k);


close(input);
close(output);
{Totusi este trist in lume}
end.