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

#define el '\n'
#define FIO ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);

using namespace std;
using namespace __gnu_pbds;

typedef long long ll;
typedef long double ld;
typedef complex<ld> pt;
typedef unsigned long long ull;

template<typename T, typename X>
using hashTable = gp_hash_table<T, X>;
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T>
using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;

// mt19937_64 for long long
mt19937 rng(std::chrono::system_clock::now().time_since_epoch().count());


int n, k;
int arr[500];
int dp[1 << 20];

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

    int &ret = dp[msk];
    if(~ret)
        return ret;

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

        if(lst != arr[i])
            c[arr[i]]++;
        lst = arr[i];
    }

    ret = 1e7;
    for(int i = 0; i < k; i++)
    {
        if(msk >> i & 1)
            continue;
        ret = min(ret, solve(msk | (1 << i)) + c[i]);
    }
    return ret;
}

void doWork()
{
    string s;
    cin >> n >> k >> s;
    ::memset(dp, -1, sizeof dp);

    int id = 0;
    map<char, int> vis;
    for(int i = 0; i < s.length(); i++)
    {
        if(!vis.count(s[i]))
            vis[s[i]] = id++;
        arr[i] = vis[s[i]];
    }
    cout << solve(0);
}

signed main()
{
    FIO
    int T = 1;
//    cin >> T;
    for(int i = 1; i <= T; i++)
        doWork();
}