#include<bits/stdc++.h>
#define MOGA ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
typedef long long int ll;
typedef long double ld;
using namespace std;
int n, t = 1, k,mask;
string s;
int dp[1 << 20];
vector<int>v(26, -1);
int f_ans(int cmask) {
    if (cmask==mask)
        return 0;
    int& rsl = dp[cmask];
    vector<int>c(20);
    if (rsl == -1) {
        rsl = 1e6;
        char pre = 'A';
        for(auto it:s){
            if (it == pre || (cmask & (1 << v[it - 'a'])))
                continue;
            pre = it;
            c[v[it - 'a']]++;
        }
        for (int i = 0;i < k;i++) {
            if (cmask ^ (1 << i)) {
                rsl = min(rsl, f_ans(cmask | (1 << i)) + c[i]);
            }
        }
    }
    return rsl;
}
void go() {
    cin >> n >> k >> s;
    int cnt = 0;
    mask = (1 << k) - 1;
    memset(dp, -1, sizeof dp);
    for (auto it : s)
        if (v[it - 'a'] == -1)
            v[it - 'a'] = cnt++;
    cout << f_ans(0);
}
int main()
{
    MOGA
      //  cin >> t;
    for (int i = 1;i <= t;i++)
    {
        go();
        cout << '\n';
    }
    return 0;
}