var i,j,n,q,s:longint;
f,g:text;
a,b,c:array[1..1000] of longint;
v:array[1..300,1..300] of longint;
begin
assign(f,'harta.in');reset(f);
assign(g,'harta.out');rewrite(g);
read(f,n);
s:=0;
for i:=1 to n do begin
read(f,a[i],b[i]);
s:=s+a[i];
                 end;
writeln(g,s);
for i:=1 to n do begin
    for j:=1 to n do c[j]:=0;
        for j:=1 to n do begin
            if (b[j]>0) and (j<>i) then begin
                                inc(c[b[j]]);
                                v[b[j],c[b[j]]]:=j;
                                        end;
                         end;
                   for j:=n downto 1 do
                        while c[j]>0 do begin
                             q:=v[j,c[j]];
                             writeln(g,i,' ',q);
                                dec(b[q]);
                                dec(a[i]);
                                dec(c[j]);
                        end;
        end;
close(f);
close(g);
end.