#include<bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

#define ll long long
#define endl '\n'
using namespace std;
using namespace __gnu_pbds;

template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
const int N = 2e5+7, P1 = 31, P2 = 37, mod= 1e9 + 7;
int n,k;
int dp[(1<<20)+5],arr[404];
set<int> st;
string s;
int solve(int mask) {
    int tmp = mask, cnt = 0;
    while (tmp) {
        cnt += (tmp % 2);
        tmp /= 2;
    }
    if (cnt == k)
        return 0;
    int &ret = dp[mask];
    if (~ret)
        return ret;
    ret = 400;
    int lst = -1;
    vector<int> ans(k, 0);
    for (int i = 0; i < n; ++i) {
        if (mask & (1 << arr[i]))
            continue;
        if (lst != arr[i])
            ans[arr[i]]++;
        lst = arr[i];
    }
    for (int i = 0; i < k; ++i) {
        if (mask & (1 << i))
            continue;
       // cout << ans[i] << endl;
        ret = min(ret, solve(mask | (1 << i)) + ans[i]);
    }
    return ret;
}
void doWork() {
    cin >> n >> k;
    map<char,int> mp;
    int cnt =0;
    cin >> s;
    for (int i = 0; i < n; ++i) {
        if ( mp.find(s[i]) == mp.end())
            mp[s[i]] = cnt++;
        arr[i] = mp[s[i]];
    }
    memset(dp,-1,sizeof dp);
    cout << solve(0) << endl;
}


int main() {
    ios::sync_with_stdio(false);
    cout.tie(nullptr);
    cin.tie(nullptr);
//    freopen("bisector.in","r",stdin);
//    freopen("bisector.out","w",stdout);
    int t = 1;
    // cout << primes.size() << endl;
    // cin >> t;
    while (t--) {
        doWork();
    }
    return 0;
}