Identify the time complexity of the following code as constant, linear, quadratic, cubic, log(n), nlog(n) or NP-hard. import java.io.*;
import java.math.BigInteger;
import java.util.*;

public class E implements Runnable {
	public static void main (String[] args) {new Thread(null, new E(), "_cf", 1 << 28).start();}

	int n, m;
	char[] str;
	int[][] occs, cost;
	int[] dp;
	
	public void run() {
		FastScanner fs = new FastScanner();
		PrintWriter out = new PrintWriter(System.out);
		System.err.println("");

		//where's my 420???? :(
		long tot = 0;
		for(int i = 0; i < 20000; i++) tot += i;
		
		n = fs.nextInt(); m = fs.nextInt();
		byte[] str = fs.next().getBytes();
		int[] occs = new int[1<<m];
		for(int i = 0; i < n-1; i++) {
			int l1 = str[i] - 'a';
			int l2 = str[i+1] - 'a';
			occs[(1<<l1) | (1<<l2)]++;
			occs[(1<<l2) | (1<<l1)]++;
		}
		//cost[mask][v] = numPairs with v for some all bits on in mask
		int all = (1<<m)-1;
		cost = new int[m][1<<m];
		for(int i = 0; i < m; i++) {
			for(int mask = 1; mask < all; mask++) {
				if(((1<<i)&mask) > 0) continue;
				int lb = mask & (-mask);
				int trail = Integer.numberOfTrailingZeros(lb);
				int nmask = mask ^ lb;
				cost[i][mask] = cost[i][nmask]+occs[1<<i | 1<<trail];
			}
		}
		
		dp = new int[1<<m];
		for(int mask = dp.length-2; mask >= 0; mask--) {
			int addOn = 0;
			for(int nxt = 0; nxt < m; nxt++) {
				if(((1<<nxt)&mask) > 0) continue;
				addOn += cost[nxt][mask];
			}
			int res = oo;
			for(int nxt = 0; nxt < m; nxt++) {
				if(((1<<nxt)&mask) > 0) continue;
				int ret = addOn+dp[mask | (1<<nxt)];
				res = min(res, ret);
			}
			dp[mask] = res;
		}
		
		System.out.println(dp[0]>>1);
		
		out.close();
	}
	
	int oo = (int)1e9;
	int min(int a, int b) {
		if(a < b) return a;
		return b;
	}
	
	class FastScanner {
		public int BS = 1<<16;
		public char NC = (char)0;
		byte[] buf = new byte[BS];
		int bId = 0, size = 0;
		char c = NC;
		double num = 1;
		BufferedInputStream in;

		public FastScanner() {
			in = new BufferedInputStream(System.in, BS);
		}

		public FastScanner(String s) {
			try {
				in = new BufferedInputStream(new FileInputStream(new File(s)),... Complexity: