source_code: import java.util.Arrays;
  import java.util.Scanner;
  
  public class thing {
  
  	public static void main(String[] args) {
  		Scanner in = new Scanner(System.in);
  		
  		int n = in.nextInt();
  		int m = in.nextInt();
  		
  		String s = in.next();
  		
  		int[][] count = new int[m][m];
  		int[] dp = new int[1 << m];
  		
  		Arrays.fill(dp, Integer.MAX_VALUE);
  		dp[0] = 0;
  		
  		for(int i = 1; i < n; i++) {
  			int a = s.charAt(i)-'a';
  			int b = s.charAt(i-1)-'a';
  			count[a][b]++;
  			count[b][a]++;
  		}
  		
  		for(int i = 1; i < (1 << m); i++) {
  			
  			int pos = set_bits(i);
  			
  			for(int j = 0; (i >> j) != 0; j++) {
  				
  				if(((i >> j) & 1) == 0) continue;
  				
  				int sum = 0;
  				
  				for(int mask = i, y = 0; y < m; mask >>= 1, y++) {
  					if(y == j) continue;
  					if((mask & 1) == 1) sum += count[j][y];
  					else sum -= count[j][y];
  				}
  				
  				int calc = dp[i-(1<<j)] + pos*sum;
  				
  				dp[i] = Math.min(dp[i], calc);
  			}
  			
  		}
  		
  		System.out.println(dp[(1 << m)-1]);
  		
  	}
  	
  	public static int set_bits(int n) { 
  		int count = 0; 
  		while (n > 0) {
  			count += n & 1;
  			n >>= 1; 
  		}
  		return count;
  	}
  
  }
  