#include<bits/stdc++.h>
using namespace std;
#define ERROR ios::sync_with_stdio(0); cin.tie(0); cout.tie(0); fileIO();
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
void fileIO(){
    #ifndef ONLINE_JUDGE
    freopen("io\\input.txt", "r", stdin);
    freopen("io\\output.txt", "w", stdout);
    #endif
}

const ll mod = 1e9 + 7, N = 2e5 + 5;

void TestCase(){
    
    int n, k;
    string s;
    cin >> n >> k >> s;
    int a[n];
    map<char, int> toInt;
    set<char> vals;
    for(int i = 0; i < n; i++)
        vals.insert(s[i]);

    int id = 0;

    for(auto ch : vals)
        toInt[ch] = id++;
    
    for(int i = 0; i < n; i++)
        a[i] = toInt[s[i]];

    int mxMask = 1 << k;

    int clicks[mxMask][k]{};

    for(int mask = 0; mask < mxMask; mask++){
        int prev = -1;
        for(int i = 0; i < n; i++){
            if(mask & (1 << a[i])) continue;
            clicks[mask][a[i]] += (a[i] != prev);
            prev = a[i];
        }
    }

    vector<int> dp(mxMask, 1e9);
    dp[0] = 0;

    for(int mask = 0; mask < mxMask; mask++){
        for(int type = 0; type < k; type++){
            if(mask & (1 << type)) continue;
            dp[mask | (1 << type)] = min(dp[mask | (1 << type)], dp[mask] + clicks[mask][type]);
        }
    }

    cout << dp[mxMask - 1] << '\n';

}

int main() {
    
    ERROR;

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

    while(t--)
        TestCase();

}