Problem: While exploring the old caves, researchers found a book, or more precisely, a stash of mixed pages from a book. Luckily, all of the original pages are present and each page contains its number. Therefore, the researchers can reconstruct the book.

After taking a deeper look into the contents of these pages, linguists think that this may be some kind of dictionary. What's interesting is that this ancient civilization used an alphabet which is a subset of the English alphabet, however, the order of these letters in the alphabet is not like the one in the English language.

Given the contents of pages that researchers have found, your task is to reconstruct the alphabet of this ancient civilization using the provided pages from the dictionary.

Input

First-line contains two integers: n and k (1 ≤ n, k ≤ 10^3) — the number of pages that scientists have found and the number of words present at each page. Following n groups contain a line with a single integer p_i (0 ≤ n < 10^3) — the number of i-th page, as well as k lines, each line containing one of the strings (up to 100 characters) written on the page numbered p_i.

Output

Output a string representing the reconstructed alphabet of this ancient civilization. If the book found is not a dictionary, output "IMPOSSIBLE" without quotes. In case there are multiple solutions, output any of them.

Example

Input


3 3
2
b
b
bbac
0
a
aca
acba
1
ab
c
ccb


Output


acb

Solution (in JAVA):

Solution:
// package oct2020;

import java.util.*;
import java.lang.*;
import java.math.*;
import java.io.*;

public class cf_subs {
	static FastReader scn = new FastReader();
	static OutputStream out = new BufferedOutputStream(System.out);
	static long MOD = (long)1e9+7;
	static ArrayList<Integer>[] g;
	@SuppressWarnings("unchecked")
	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		int tc = 1;
		
		while(tc-->0) {
			int n =scn.nextInt(),k=scn.nextInt();
			ArrayList<String>[] book = new ArrayList[n];
			g = new ArrayList[26];
			int[] in=new int[26];
			for(int i=0;i<26;i++) {
				g[i]=new ArrayList<>();
			}
			for(int i=0;i<n;i++) {
				book[i]=new ArrayList<>();
			}
			HashSet<Character> letters = new HashSet<>();
			for(int i=0;i<n;i++) {
				int p= scn.nextInt();
				for(int j=0;j<k;j++) {
					String st  =scn.next();
					book[p].add(st);
					for(char ch:st.toCharArray()) {
						letters.add(ch);
					}
				}
			}
			ArrayList<String> flattened_book = new ArrayList<>();
			for(int i=0;i<n;i++) {
				for(String st : book[i]) {
					flattened_book.add(st);
				}
			}
			
			for(int i=1;i<flattened_book.size();i++) {
				String s1 = flattened_book.get(i-1);
				String s2 = flattened_book.get(i);
				
				if(s2.lastIndexOf(s1, 0)==0) {
					continue;
				}
				if(s1.lastIndexOf(s2,0)==0) {
					out.write("IMPOSSIBLE\n".getBytes());
					out.close();
					return;
				}
				
				int diff_pos = 0;
				while(s1.charAt(diff_pos)==s2.charAt(diff_pos)) {
					diff_pos++;
				}
				
				int u = s1.charAt(diff_pos)-'a';
				int v = s2.charAt(diff_pos)-'a';
				g[u].add(v);
				in[v]++;
				
			}
			ArrayDeque<Integer> q =new ArrayDeque<>();
			for(int i=0;i<26;i++) {
				if(letters.contains((char)(i+'a'))&&in[i]==0) {
					q.add(i);
				}
			}
			ArrayList<Integer> res = new ArrayList<>();
			while(!q.isEmpty()) {
				int u = q.pollFirst();
				res.add(u);
				for(int v:g[u]) {
					if(--in[v]==0) {
						q.add(v);
					}
				}
			}
			for(int i=0;i<26;i++) {
				if(in[i]>0) {
					res =new ArrayList<>();
					break;
				}
			}
			if(res.size()==0) {
				out.write("IMPOSSIBLE\n".getBytes());
				out.close();
				return;
			}
			
			for(int i:res) {
				out.write(((char)(i+'a')+"").getBytes());
			}
			out.write("\n".getBytes());
		}
		out.close();
	}
//	static class Page implements Comparable<Page>{
//		int pno;
//		ArrayList<String> strs;
//		ArrayList<Page> kids;
//		public Page(int pno) {
//			this.pno=pno;
//			this.strs = new ArrayList<>();
//			this.kids = new ArrayList<>();
//		}
//		public int compareTo(Page o) {
//			return this.pno-o.pno;
//		}
//	}
	static class Edge implements Comparable<Edge>{
		int u,v,w;
		public Edge(int u, int v, int w) {
			this.u=u;this.v =v;this.w =w;
		}
		public int compareTo(Edge o) {
			return this.w-o.w;
		}
		public String toString() {
			return "["+u+','+v+','+w+']';
		}
	}
	static class Pair<S extends Comparable<S>,T extends Comparable<T>> implements Comparable<Pair<S,T>>{
		S first;
		T second;
		public Pair() {
			this.first=null;
			this.second =null;
		}
		public Pair(S first) {
			this.first = first;
			this.second =null;
		}
		public Pair(S first, T second) {
			this.first = first;
			this.second = second;
		}
		public int compareTo(Pair<S,T> o) {
			if(this.first.compareTo(o.first)==0) {
				return this.second.compareTo(o.second);
			}
			return this.first.compareTo(o.first);
		}
		public String toString() {
			return "{"+this.first.toString()+","+this.second.toString()+"}";
		}
	}
	static int log(long N) {
		// TODO Auto-generated method stub
		return 63- Long.numberOfLeadingZeros(N);
	}
	static int lowerBound(int[] a, int x,int lf,int rg) {
		int l = lf-1, r = rg+1;
		while (r - l > 1) {
			int c = (l + r) / 2;
			if (a[c] < x) {
				//if (a[c] > x) {
				l = c;
			} else {
				r = c;
			}
		}
		return r;
	}
	static int upperBound(int[] a, int x,int lf,int rg) {
		int l = lf-1, r = rg+1;
		while (r - l > 1) {
			int c = (l + r) / 2;
			if (a[c] <= x) {
				l = c;
			} else {
				r = c;
			}
		}
		return r;
	}
 
	static int lowerBound(long[] a, long x) {
		int l = -1, r = a.length;
		while (r - l > 1) {
			int c = (l + r) / 2;
			if (a[c] < x) {
				l = c;
			} else {
				r = c;
			}
		}
		return r;
	}
 
	static int upperBound(long[] a, long x) {
		int l = -1, r = a.length;
		while (r - l > 1) {
			int c = (l + r) / 2;
			if (a[c] <= x) {
				l = c;
			} else {
				r = c;
			}
		}
		return r;
	}
 
	static int lowerBound(double[] a, double x) {
		int l = -1, r = a.length;
		while (r - l > 1) {
			int c = (l + r) / 2;
			if (a[c] < x) {
				l = c;
			} else {
				r = c;
			}
		}
		return r;
	}
 
	static int upperBound(double[] a, double x) {
		int l = -1, r = a.length;
		while (r - l > 1) {
			int c = (l + r) / 2;
			if (a[c] <= x) {
				l = c;
			} else {
				r = c;
			}
		}
		return r;
	}
 
	static <T> int lowerBound(List<T> ls, T x) throws RuntimeException {
		if (ls.size() == 0)
			return -1;
		if (ls.get(0) instanceof Integer) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Integer) t1).compareTo((Integer) t2) >= 0 ? 1 : -1);
		} else if (ls.get(0) instanceof Long) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Long) t1).compareTo((Long) t2) >= 0 ? 1 : -1);
		} else if (ls.get(0) instanceof Double) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Double) t1).compareTo((Double) t2) >= 0 ? 1 : -1);
		} else {
			System.err.println(
					String.format("%s:Arey Maa chudi padi hai", Thread.currentThread().getStackTrace()[1].getMethodName()));
			throw new RuntimeException();
		}
	}
 
	static <T> int upperBound(List<T> ls, T x) throws RuntimeException {
		if (ls.size() == 0)
			return -1;
		if (ls.get(0) instanceof Integer) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Integer) t1).compareTo((Integer) t2) > 0 ? 1 : -1);
		} else if (ls.get(0) instanceof Long) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Long) t1).compareTo((Long) t2) > 0 ? 1 : -1);
		} else if (ls.get(0) instanceof Double) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Double) t1).compareTo((Double) t2) > 0 ? 1 : -1);
		} else {
			System.err.println(
					String.format("%s:Arey Maa chudi padi hai", Thread.currentThread().getStackTrace()[1].getMethodName()));
			throw new RuntimeException();
		}
	}
 
	static <T> int rupperBound(List<T> ls, T x) throws RuntimeException {
		if (ls.size() == 0)
			return -1;
		if (ls.get(0) instanceof Integer) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Integer) t1).compareTo((Integer) t2) < 0 ? 1 : -1);
		} else if (ls.get(0) instanceof Long) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Long) t1).compareTo((Long) t2) < 0 ? 1 : -1);
		} else if (ls.get(0) instanceof Double) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Double) t1).compareTo((Double) t2) < 0 ? 1 : -1);
		} else {
			System.err.println(
					String.format("%s:Arey maa chudi padi hai", Thread.currentThread().getStackTrace()[1].getMethodName()));
			throw new RuntimeException();
		}
	}
 
	static <T> int rlowerBound(List<T> ls, T x) {
		if (ls.size() == 0)
			return -1;
		if (ls.get(0) instanceof Integer) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Integer) t1).compareTo((Integer) t2) <= 0 ? 1 : -1);
		} else if (ls.get(0) instanceof Long) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Long) t1).compareTo((Long) t2) <= 0 ? 1 : -1);
		} else if (ls.get(0) instanceof Double) {
			return ~Collections.binarySearch(ls, x, (t1, t2) -> ((Double) t1).compareTo((Double) t2) <= 0 ? 1 : -1);
		} else {
			System.err.println(
					String.format("%s:Arey maa chudi padi hai", Thread.currentThread().getStackTrace()[1].getMethodName()));
			throw new RuntimeException();
		}
	}
	public static int[] merge(int[] one,int[] two){
		int[] res = new int[one.length+two.length];
		int i = 0;
		int j = 0;
		int k = 0;
		
		while(i<one.length&&j<two.length){
			if(one[i]<two[j]){
				res[k] = one[i];
				i++;
				k++;
			}
			else{
				res[k] = two[j];
				j++;
				k++;
			}
		}
		
		if(i==one.length){
			while(j<two.length){
				res[k] = two[j];
				j++;
				k++;
			}
		}
		else{
			while(i<one.length){
				res[k] = one[i];
				k++;
				i++;
			}
		}
		return res;
	}
	public static int[] mergesort(int[] arr, int l, int r){
		if(l==r){
			int[] br = new int[1];
			br[0] = arr[l];
			return br;
		}
		int mid = (l+r)/2;
		int[] fh = mergesort(arr,l,mid);
		int[] sh = mergesort(arr,mid+1,r);
		
		return merge(fh,sh);
	}
	static class FastReader 
    { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() 
        { 
            br = new BufferedReader(new
                     InputStreamReader(System.in)); 
        } 
  
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() 
        { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    }

}