#include <bits/stdc++.h>

#define el '\n'

typedef long long ll;
typedef long double ld;

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

using namespace std;

const int N = 400 + 5, INF = 1e9;

int n, k;
int a[N], dp[1 << 20];

int solve(int msk) {
    if (msk == (1 << k) - 1)
        return 0;

    int &ret = dp[msk];

    if (~ret)
        return ret;

    vector<int> cst(k);
    for (int i = 0, lst = -1; i < n; i++) {
        if (msk & (1 << a[i]))
            continue;

        if (a[i] != lst)
            cst[a[i]]++;

        lst = a[i];
    }

    ret = INF;

    for (int bit = 0; bit < k; bit++) {
        if (msk & (1 << bit))
            continue;

        ret = min(ret, solve(msk | (1 << bit)) + cst[bit]);
    }

    return ret;
}

void testCase() {
    string s;
    cin >> n >> k >> s;

    int id = 0;
    vector<int> ids(26, -1);
    for (int i = 0; i < n; i++) {
        if (ids[s[i] - 'a'] == -1)
            ids[s[i] - 'a'] = id++;

        a[i] = ids[s[i] - 'a'];
    }

    memset(dp, -1, sizeof dp);

    cout << solve(0);
}

signed main() {
    Beevo

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

    while (t--)
        testCase();

    return 0;
}