#include <bits/stdc++.h>
using namespace std;
const int maxn = , inf = 0x3f3f3f;
struct Edge {
int from, to, cap, flow;
}; struct Dinic {
int n, m, s, t;
vector<Edge> edges;
vector<int> G[maxn];
bool vis[maxn];
int d[maxn];
int cur[maxn]; void AddEdge(int from, int to, int cap) {
edges.push_back((Edge){from, to, cap, });
edges.push_back((Edge){to, from, , });
m = edges.size();
G[from].push_back(m-);
G[to].push_back(m-);
}
bool bfs() {
memset(vis, , sizeof(vis));
queue<int> que;
que.push(s);
d[s] = ;
vis[s] = true;
while (!que.empty()) {
int x = que.front(); que.pop();
for (int i = ; i < G[x].size(); ++i) {
Edge& e = edges[G[x][i]];
if (!vis[e.to] && e.cap > e.flow) {
vis[e.to] = true;
d[e.to] = d[x] + ;
que.push(e.to);
}
}
}
return vis[t];
}
int dfs(int x, int a) {
if (x == t || a == ) return a;
int flow = , f;
for (int& i = cur[x]; i < G[x].size(); ++i) {
Edge& e = edges[G[x][i]];
if (d[x] + == d[e.to] && (f = dfs(e.to, min(a, e.cap-e.flow))) > ) {
e.flow += f;
edges[G[x][i]^].flow -= f;
flow += f;
a -= f;
if (a == ) break;
}
}
return flow;
}
int maxflow(int s, int t) {
this->s = s; this->t = t;
int flow = ;
while (bfs()) {
memset(cur,,sizeof(cur));
flow += dfs(s,inf);
}
return flow;
}
}dinic;
int main() {
int m, n; scanf("%d%d",&m,&n);
while (true) {
int i, j; scanf("%d%d",&i,&j);
if (i == - && j == -) break;
dinic.AddEdge(i,j,);
}
int s = n+, t = n+;
for (int i = ; i <= m; ++i) {
dinic.AddEdge(s,i,);
}
for (int i = m+; i <= n; ++i) {
dinic.AddEdge(i,t,);
}
int ans = dinic.maxflow(s,t);
if (ans == ) puts("No Solution!");
else {
printf("%d\n",ans);
for (int i = ; i < dinic.edges.size(); ++i) {
if (dinic.edges[i].from == s) continue;
if (dinic.edges[i].to == t) continue;
if (dinic.edges[i].flow == ) {
printf("%d %d\n",dinic.edges[i].from,dinic.edges[i].to);
}
}
}
return ;
}