#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<queue>
#include<vector>
using namespace std;
const int INF = 0x3f3f3f3f;
const int MAXN = 1e5 + 5;
struct Edge
{
int to, next;
long long w;
}edg[MAXN];
int head[MAXN], dep[MAXN], pre[MAXN];
int cnt = 0;
void Init()
{
memset(head, -1, sizeof(head));
cnt = 0;
}
int Find(int root)
{
if(pre[root] != root)
{
pre[root] = Find(pre[root]);
}
return pre[root];
}
void Union(int x, int y)
{
int Fx = Find(x);
int Fy = Find(y);
if(Fx != Fy)
pre[Fx] = Fy;
}
inline void Add_edge(int u, int v, int w)
{
edg[cnt].w = w;
edg[cnt].to = v;
edg[cnt].next = head[u];
head[u] = cnt++;
edg[cnt].to = u;
edg[cnt].w = 0;
edg[cnt].next = head[v];
head[v] = cnt++;
}
bool BFS(int s, int t)
{
queue<int> q;
while(!q.empty()) q.pop();
memset(dep, 0, sizeof(dep));
dep[s] = 1;
q.push(s);
while(!q.empty())
{
int u = q.front(); q.pop();
for(int i = head[u]; i != -1; i = edg[i].next)
{
int v = edg[i].to;
int w = edg[i].w;
if(w && dep[v] == 0)
{
dep[v] = dep[u] + 1;
q.push(v);
if(v == t)
return true;
}
}
}
return false;
}
int DFS(int s, long long mw, int t)
{
if(s == t)
return mw;
long long res = 0, k;
for(int i = head[s]; i != -1 && res < mw; i = edg[i].next)
{
int v = edg[i].to;
int w = edg[i].w;
if(dep[v] == dep[s] + 1 && w)
{
k = DFS(v, min(mw - res, edg[i].w), t);
if(k == 0)
dep[v] = 0;
edg[i].w -= k;
edg[i^1].w += k;
res += k;
}
}
return res;
}
long long Dinic(int s, int t)
{
int maxflow = 0;
while(BFS(s, t))
{
while(long long d = DFS(s, INF, t))
{
maxflow += d;
}
}
return maxflow;
}
int main()
{
int n, m;
scanf("%d %d", &m, &n);
int s = 0, t = n + 1;
Init();
int x, y;
while(scanf("%d %d", &x, &y) && x != -1 && y != -1)
{
Add_edge(x, y, INF);
}
for(int i = 1; i <= m; ++i)
{
Add_edge(s, i, 1);
}
for(int i = m + 1; i <= n; ++i)
{
Add_edge(i, t, 1);
}
int ans = Dinic(s, t);
if(ans == 0)
printf("No Solution!\n");
else
printf("%d\n", ans);
for(int i = 0; i <= cnt; i += 2)
{
if(edg[i].to != s && edg[i^1].to != s)
{
if(edg[i].to != t && edg[i^1].to != t)
{
if(edg[i ^ 1].w != 0)
{
printf("%d %d\n", edg[i^1].to, edg[i].to);
}
}
}
}
}