#include <fstream>
#include <queue>
#include <unordered_map>
#include <vector>
#include <limits>


using namespace std;

class Graph {
private:
	int _nodes;
	vector<unordered_map<int, int>> _residual_graph;
	vector<int> _bfs_tree;
	vector<bool> _visited;

	bool BFS(const int start, const int dest) {

		queue<int> node_queue;
		_visited = vector<bool>(_nodes);
		_bfs_tree = vector<int>(_nodes);

		_bfs_tree[start] = -1;
		_visited[start] = true;
		node_queue.push(start);

		while (!node_queue.empty()) {
			const int node = node_queue.front();
			node_queue.pop();

			for (auto& edge : _residual_graph[node]) {

				if (!_visited[edge.first] && edge.second > 0) {
					_visited[edge.first] = true;
					_bfs_tree[edge.first] = node;
					node_queue.push(edge.first);
				}
			}
		}
		return _visited[dest];
	}
public:
	Graph(int nodes, vector<unordered_map<int, int>> adj_list) :
		_nodes(nodes),
		_residual_graph(move(adj_list))
	{
		for (int i = 0; i < _nodes; ++i) {
			for (const auto& edge : _residual_graph[i]) {
				if (_residual_graph[edge.first].find(i) == _residual_graph[edge.first].end())
					_residual_graph[edge.first][i] = 0;
			}
		}
	}

	int maxFlow(const int source, const int dest) {
		int max_flux = 0;
		while (BFS(source, dest)) {
			for (const auto& edge : _residual_graph[dest]) {

				int node = edge.first;
				if (_residual_graph[node][dest] == 0 || !_visited[node])
					continue;

				int flux = numeric_limits<int>::max();
				flux = min(flux, _residual_graph[node][dest]);

				while (_bfs_tree[node] != -1) {
					const int father = _bfs_tree[node];
					flux = min(flux, _residual_graph[father][node]);
					if (flux == 0)
						break;
					node = father;
				}

				if (flux == 0)
					continue;
				max_flux += flux;

				node = edge.first;
				_residual_graph[node][dest] -= flux;
				_residual_graph[dest][node] += flux;
				while (_bfs_tree[node] != -1) {
					const int father = _bfs_tree[node];
					_residual_graph[father][node] -= flux;
					_residual_graph[node][father] += flux;
					node = father;
				}
			}
		}
		return max_flux;
	}
	vector<pair<int, int>> getGraph(const int nodes) {

		vector<pair<int, int>> edges;

		maxFlow(0, _nodes - 1);

		for (int i = 1; i <= nodes; ++i) {
			for (const auto& edge : _residual_graph[i]) {
				int j = edge.first;
				if (j != 0 && _residual_graph[i][edge.first] == 0) {
					edges.emplace_back(i, j - nodes);
				}
			}
		}
		return edges;

	}
};

int main()
{
	ifstream in("harta.in");

	int nodes;
	in >> nodes;

	int internal_nodes = 2 * nodes + 2;
	vector<unordered_map<int, int>> adj_list(internal_nodes);

	for (int i = 1; i <= nodes; ++i) {
		int degree_out, degree_in;
		in >> degree_out >> degree_in;

		adj_list[0][i] = degree_out;
		adj_list[i + nodes][internal_nodes - 1] = degree_in;
		for(int j = nodes + 1; j < internal_nodes - 1; ++j){
			if (j - nodes == i)
				continue;
			adj_list[i][j] = 1;
		}
	}
	in.close();


	ofstream out("harta.out");

	Graph fromDegrees(internal_nodes, adj_list);

	auto sol = fromDegrees.getGraph(nodes);
	out << sol.size()<<"\n";
	for(const auto& edge : sol){
		out << edge.first << " " << edge.second << "\n";
	}
	out.close();

	return 0;
}