Cut Bridges

// NgJaBach: Forever Meadow <3

#include<bits/stdc++.h>

using namespace std;
typedef long long int ll;
typedef unsigned long long ull;
#define pb push_back
#define pob pop_back
#define mp make_pair
#define upb upper_bound
#define lwb lower_bound
#define bend(a) a.begin(),a.end()
#define rev(x) reverse(bend(x))
#define mset(a) memset(a, 0, sizeof(a))
#define fi first
#define se second
#define gcd __gcd
#define getl(s) getline(cin, s);
#define setpre(x) fixed << setprecision(x)
#define endl '\n'
const int N=200050,M=1000000007;
const ll INF=1e18+7;
int cnt=0,dis[N],low[N];
vector<int>gay[N];
vector<pair<int,int> >bridges;
void dfs(int u,int p){
    low[u]=dis[u]=cnt++;
    for(auto v:gay[u]){
        if(v!=p){
            if(dis[v]!=-1){
                low[u]=min(low[u],dis[v]);
            }
            else{
                dfs(v,u);
                low[u]=min(low[u],low[v]);
                if(dis[v]<=low[v]){
                    bridges.pb({min(u,v),max(u,v)});
                }
            }
        }
    }
    return;
}
int main(){
    ios_base::sync_with_stdio(NULL); cin.tie(nullptr); cout.tie(nullptr);
//    freopen(".inp","r",stdin);
//    freopen(".out","w",stdout);
    int n,m,x,y;
    cin>>n>>m;
    while(m--){
        cin>>x>>y;
        gay[x].pb(y);
        gay[y].pb(x);
    }
    for(int i=1;i<=n;++i) dis[i]=-1;
    for(int i=1;i<=n;++i){
        if(dis[i]==-1) dfs(i,i);
    }
    n=bridges.size();
    sort(bend(bridges));
    cout<<n<<endl;
    for(int i=0;i<n;++i) cout<<bridges[i].fi<<" "<<bridges[i].se<<endl;
    return 0;
}
/*
==================================+
INPUT:                            |
------------------------------    |

------------------------------    |
==================================+
OUTPUT:                           |
------------------------------    |

------------------------------    |
==================================+
*/