#include <bits/stdc++.h>
using namespace std;

ifstream in("harta.in");
ofstream out("harta.out");
int n, x, y, s, mx, poz;

struct Nod {
    int in;
    int nod;
    int out;
    Nod(int a, int b, int c) : in(a), nod(b), out(c) {}
    bool operator < (const Nod& other) const
    {
        if (in > other.in) 
            return true;
        if (in < other.in) 
            return false;
        else
            return out > other.out;
    }
};
vector<Nod> drum;

int main()
{
    in >> n;
    drum.push_back({0, 0, 0});
    
    for(int i = 1; i <= n; i++)
    {
        in >> x >> y;
        Nod nod(y, i, x);
        drum.push_back(nod);
        s += y;
    }
    
    out << s << endl;
    
    while(n)
    {
        sort(drum.begin() + 1, drum.end());
        n--;
        mx = 0; poz = 0;
        
        for (int i = 1; i < drum.size(); i++)
        {
            if (drum[i].out > mx)
            {
                mx = drum[i].out;
                poz = i;
            }
        }
        
        for (int i = 1; i <= mx + 1; i++)
        {
            if (i != poz && drum[poz].out > 0)
            {
                drum[i].in--;
                drum[poz].out--;
                out << drum[poz].nod << " " << drum[i].nod << endl;
            }
        }
    }
    
    return 0;
}