#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);
                }
            }
        }
    }
}