You are given a graph with n nodes and m directed edges. One lowercase letter is assigned to each node. We define a path's value as the number of the most frequently occurring letter. For example, if letters on a path are "abaca", then the value of that path is 3. Your task is find a path whose value is the largest.

Input

The first line contains two positive integers n, m (1 ≤ n, m ≤ 300 000), denoting that the graph has n nodes and m directed edges.

The second line contains a string s with only lowercase English letters. The i-th character is the letter assigned to the i-th node.

Then m lines follow. Each line contains two integers x, y (1 ≤ x, y ≤ n), describing a directed edge from x to y. Note that x can be equal to y and there can be multiple edges between x and y. Also the graph can be not connected.

Output

Output a single line with a single integer denoting the largest value. If the value can be arbitrarily large, output -1 instead.

Examples

Input

5 4
abaca
1 2
1 3
3 4
4 5


Output

3


Input

6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4


Output

-1


Input

10 14
xzyzyzyzqx
1 2
2 4
3 5
4 5
2 6
6 8
6 5
2 10
3 9
10 9
4 6
1 10
2 8
3 7


Output

4

Note

In the first sample, the path with largest value is 1 → 3 → 4 → 5. The value is 3 because the letter 'a' appears 3 times.
import java.io.*;
import java.lang.reflect.Array;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.math.RoundingMode;
import java.text.DecimalFormat;
import java.util.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;


public class Main
{	
   static long mod=(long)(1e+9 + 7);
	
   //static long mod=(long)998244353;
   static int[] sieve;
   static ArrayList<Integer> primes;
   static PrintWriter out;
   static fast s;
   static StringBuilder fans;
   
   static int x_dir[];
   static int y_dir[];
   static int flag=1;
   
   public static void dfs1(int start,ArrayList<Integer>[] graph,int[] visited,int[] stack)
   {
   	if(stack[start]==1) flag=1;
   	if(visited[start]==1) return;
   	
   	visited[start]=1;
   	stack[start]=1;
   	
   	for(int i=0;i<graph[start].size();i++)
   		dfs1(graph[start].get(i),graph,visited,stack);
   	
   	stack[start]=0;
   }
   
   public static void solve()
   {
   	int n=s.nextInt();
   	int m=s.nextInt();
   	
   	char a[]=new char[n+1];
   	String str=s.nextLine();
   	for(int i=0;i<str.length();i++) a[i+1]=str.charAt(i);
   	
   	ArrayList<Integer> graph[]=new ArrayList[n+1];
   	for(int i=1;i<=n;i++) graph[i]=new ArrayList<Integer>();
   	
   	int in[]=new int[n+1];
   	
   	for(int i=1;i<=m;i++)
   		{
   			int u=s.nextInt();
   			int v=s.nextInt();
   			graph[u].add(v);
   			in[v]++;
   		}
   	
   	flag=0;
   	int stack[]=new int[n+1];
   	int visited[]=new int[n+1];
   	
   	for(int i=1;i<=n;i++)
   		{
   			if(visited[i]==0) dfs1(i,graph,visited,stack);
   		}
   	
   	if(flag==1) fans.append("-1");
   	else
   		{
   			LinkedList<Integer> qu1=new LinkedList<Integer>();
   			
   			int dp[][]=new int[n+1][26];
   			
   			for(int i=1;i<=n;i++) 
   				if(in[i]==0) 
   					{
   						qu1.add(i);
   						dp[i][a[i]-'a']++;
   					}
   			
   			int max=0;
   			
   			while(qu1.size()>0)
   				{
   					int cur=qu1.pollFirst();
   			
   					for(int i=0;i<graph[cur].size();i++)
   						{
   							int v=graph[cur].get(i);
   							int ind=a[v]-'a';
   							
   							for(int j=0;j<26;j++)
   									dp[v][j]=Math.max(dp[v][j], dp[cur][j]+((j==ind)?1:0));
   							
   							in[v]--;
   							if(in[v]==0)
   								qu1.add(v);
			   					
   						}
   				}
   			
   			for(int i=1;i<=n;i++)
   				for(int j=0;j<26;j++)
   					max=Math.max(max, dp[i][j]);
   			fans.append(max);
   		}
   }
   
   public static void main(String[] args) throws java.lang.Exception
   {	  	
	     s = new fast();
	     
	     out=new PrintWriter(System.out);
	     fans=new StringBuilder("");
		  
	     int t=1;
	     
	     while(t>0)
	    	 {
	    		 solve();
	    		 t--;
	    	 }
	    
	    out.println(fans);
	    out.close();
   }
  
 
 
static class fast {   
	
	   private InputStream i;
		private byte[]  buf = new byte[1024];
		private int curChar;
		
		private int numChars;
		
		//Return floor log2n
		public static long log2(long bits) // returns 0 for bits=0
			{
			    int log = 0;
			    if( ( bits & 0xffff0000 ) != 0 ) { bits >>>= 16; log = 16; }
			    if( bits >= 256 ) { bits >>>= 8; log += 8; }
			    if( bits >= 16  ) { bits >>>= 4; log += 4; }
			    if( bits >= 4   ) { bits >>>= 2; log += 2; }
			    return log + ( bits >>> 1 );
			}
		
		public static boolean next_permutation(int a[])
		   {
			   int i=0,j=0;int index=-1;	   
			   int n=a.length;
			   
			   for(i=0;i<n-1;i++)
				   if(a[i]<a[i+1]) index=i;
			   
			   if(index==-1) return false;
			   i=index;
 
			  
			   for(j=i+1;j<n && a[i]<a[j];j++);
 
			   int temp=a[i];
			   a[i]=a[j-1];
			   a[j-1]=temp;
			   	   
			   for(int p=i+1,q=n-1;p<q;p++,q--)
			   {
				   temp=a[p];
				   a[p]=a[q];
				   a[q]=temp;
			   }
			   
			   return true;
		   }
		   
		public static void sieve(int size)
		{
			sieve=new int[size+1];
			
			primes=new ArrayList<Integer>();
			
			sieve[1]=1;
			
			for(int i=2;i*i<=size;i++)
			{
				if(sieve[i]==0)
				 {
					for(int j=i*i;j<size;j+=i) {sieve[j]=1;}
				 }
			}
			
			for(int i=2;i<=size;i++)
			{
				if(sieve[i]==0) primes.add(i);
			}
		}
		
		public static long pow(long a, long p){
		    long o = 1;
		    for(; p>0; p>>=1){
		        if((p&1)==1)o = mul(o, a);
		        a = mul(a, a);
		    }
		    return o;
		}
		
		public static long add(long... a){
		    long o = 0;
		    for(long x:a)o = (o+mod+x)%mod;
		    return o;
		}
		
		public static long mul(long... a){
		    long p = 1;
		    for(long x:a)p = (mod+(p*x)%mod)%mod;
		    return p;
		}
		
		public static long mod_inv(long n)
		{
			return pow(n,mod-2);
		}
		   
		public long gcd(long r,long ans)
		{
			if(r==0) return ans;
			return gcd(ans%r,r);
		}
		
		public fast() {
			this(System.in);
		}
		public fast(InputStream is) {
			i = is;
		}
		public int read() {
			if (numChars == -1)
				throw new InputMismatchException();
			if (curChar >= numChars) {
				curChar = 0;
				try {
					numChars = i.read(buf);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (numChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}
		public String nextLine() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isEndOfLine(c));
			return res.toString();
		}
		public String nextString() {
     		int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}
		public long nextLong() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			long res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
		public int nextInt() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			int res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
		public boolean isSpaceChar(int c) {
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}
		public boolean isEndOfLine(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}
 
	}	
}