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