#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#define ll long long
template<typename T>
using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;

typedef long double ld;


#define lmao ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

const int N = 1e5+5 , M = 1e4+5 , MOD = 1e9+7 , LOG = 20 , Msk = 21;

int MaskValues[1 << Msk][25];

ll dp[1 << Msk];
ll actualMsk , n , k , cnt = 0;

ll slv(ll curMask){

    if(curMask == actualMsk)
        return 0;
    ll &ret = dp[curMask];
    if(~ret)
        return ret;
    ret = 1e9;
    for(int i = 0 ; i < k ; i++){
        if(curMask & (1 << i))
            continue;
        ret = min(ret , slv(curMask | (1 << i)) + MaskValues[curMask][i]);
    }
    return ret;
}

void testCase() {
    string s;
    cin >> n >> k >> s;
    memset(dp , -1, sizeof dp);
    for(int i = 0 ; i < k ; i++)
        actualMsk |= (1 << i);

    map<char,char>visited;
    for(auto &it:s){
        if(visited.count(it))
            it = visited[it];
        else
            visited[it] = (cnt + 'a'), it = (cnt + 'a') , cnt++;
    }
    for(int i = 0 ; i < (1 << k) ; i++){
        char prev = '?';
        for(int j = 0 ; j < n ; j++){
            if((1 << (s[j]-'a') & i))
                continue;
            MaskValues[i][s[j] - 'a'] += (s[j] != prev);
            prev = s[j];
        }
    }
    cout << slv(0);
}

signed main() {

    lmao
    int t = 1;
//    cin >> t;

    while (t--)
        testCase();
}