#include <bits/stdc++.h>

using namespace std;
const int N = (1 << 20) + 10;
int dp[N];
int n, k;
vector<int> v;
vector<int> frq[21];

int rec(int msk) {
    if (msk == 0) {
        return 0;
    }
    auto &ans = dp[msk];
    if (~ans)
        return ans;
    ans = 1e9;
    vector<int> cnt(k), last(k, -2);
    int last0 = -1;
    for (int i = 0; i < n; i++) {
        if (((1 << v[i]) & msk) == 0)
            last0 = i;
        else {
            cnt[v[i]] += (last0 > last[v[i]]);
            last[v[i]] = i;
        }
    }
    for (int i = 0; i < k; i++) {
        if (!(msk & (1 << i)))
            continue;

        ans = min(ans, rec(msk ^ (1 << i)) + cnt[i]);
    }
    return ans;
}

void tc() {
    memset(dp, -1, sizeof dp);
    cin >> n >> k;
    map<char, int> mp;
    string s;
    cin >> s;
    for (auto &i: s) {
        if (mp.find(i) == mp.end())
            mp[i] = mp.size();
        v.push_back(mp[i]);
    }
    cout << rec((1 << k) - 1) << '\n';
}

signed main() {
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    int t = 1;
    //    cin >> t;

    while (t--) {
        tc();
    }

    return 0;
}