#include<iostream>
#include<stdio.h>
#include<queue>
#include<vector>
using namespace std;
int n, m, used[1001];
vector<int> a[1001];
void in()
{ scanf("%d %d", &n, &m);
  for (int i = 0; i < m; i++)
  { int v1, v2;
    scanf("%d %d", &v1, &v2);
    a[v1 - 1].push_back(v2 - 1);
    a[v2 - 1].push_back(v1 - 1);
  }
}

void bfs(int i)
{ queue<int> q;
  int br1 = 1, br2 = 0, level = 0; //br1 holds the remaining number of vertices from the current level and in br2 we count the number of vertices in the next level.
  
  used[i] = 1;
  q.push(i);
  
  printf("level %d:", level);
  
  while (!q.empty())
  { if (br1 == 0)
    { br1 = br2;
      br2 = 0;
      level++;
      printf("\nlevel %d:", level);
    }
    
    int g = q.front();
    q.pop();
    
    br1--;
    
    printf(" %d", g + 1);
    
    int s = a[g].size();
    for (int j = 0; j < s; j++)
      if (used[a[g][j]] == 0)
      { used[a[g][j]] = 1;
        q.push(a[g][j]);
        br2++;
      }
  }
  
  printf("\n");
}

/*void bfs(int i) //Here we use one queue which elements are of type pair<int, int> - the first value in the pair is vertex and the second value in the pair is the level of that vertex. We use more memory here.
{ queue<pair<int, int> > q;
  int lastLevel = 0;
  
  used[i] = 1;
  q.push(make_pair(i, 0));
  
  printf("level %d:", q.front().second);
  
  while (!q.empty())
  { pair<int, int> g = q.front();
    q.pop();
    
    if (g.second == lastLevel)
      printf(" %d", g.first + 1);
    
    else
    { lastLevel = g.second;
      printf("\nlevel %d: %d", g.second, g.first + 1);
    }
    
    int s = a[g.first].size();
    for (int j = 0; j < s; j++)
      if (used[a[g.first][j]] == 0)
      { used[a[g.first][j]] = 1;
        q.push(make_pair(a[g.first][j], g.second + 1));
      }
  }
  
  printf("\n");
}*/

int main()
{ 
  in();
  bfs(0);
  
  return 0;
}