var i,j,n:longint;
    f1,f2:text;
    a,b:array[1..1000] of longint;
    m:array[1..110,1..110] of integer;
begin
        assign(f1,'harta.in');
        reset(f1);
        assign(f2,'harta.out');
        rewrite(f2);
        read(f1,n);
        for i:=1 to n do
                read(f1,a[i],b[i]);
        for i:=1 to n do
        begin
                j:=i+1;
                while a[i]>0 do
                begin
                        if b[j]>0 then
                        begin
                                dec(a[i]);
                                dec(b[j]);
                                m[i,j]:=1;
                        end;
                        j:=(j+1)mod n;
                        if j=0 then inc(j);
                end;
        end;
        a[1]:=0;
        for i:=1 to n do
                for j:=1 to n do
                        if m[i,j]=1 then inc(a[1]);
        writeln(f2,a[1]);
        for i:=1 to n do
                for j:=1 to n do
                        if m[i,j]=1 then writeln(f2,i,' ',j);
        close(f1);
        close(f2);
end.