1 #include<cstdio>
 2 #include<cstring>
 3 #include<iostream>
 4 using namespace std;
 5 const int sz = 100010;
 6 int n, m, num = 0, ans = 0;
 7 int head[sz], match[sz];
 8 bool vis[sz];
 9 struct edge {
10     int nxt, to;
11 }e[sz << 1];
12 void add(int from, int to) {
13     e[++num].nxt = head[from];
14     e[num].to = to;
15     head[from] = num;
16 }
17 bool dfs(int u) {
18     for(int i = head[u]; i; i = e[i].nxt) {
19         int v = e[i].to;
20         if(!vis[v]) {
21             vis[v] = 1;
22             if(!match[v] || dfs(match[v])) {
23                 match[v] = u;
24                 return true;
25             }
26         }
27     }
28     return false;
29 }
30 int main() {
31     scanf("%d%d", &n, &m);
32     while(1) {
33         int u, v;
34         scanf("%d%d", &u, &v);
35         if(u == -1 && v == -1) break;
36         add(u, v);
37     }
38     for(int i = 1; i <= n; i++) {
39         memset(vis, 0, sizeof(vis));
40         if(dfs(i)) ans++;
41     }
42     printf("%d\n", ans);
43     for(int i = 1; i <= m; i++)
44         if(match[i]) printf("%d %d\n", match[i], i);
45     return 0;
46 }