#include <bits/stdc++.h>

#define INF 1000000000
#define MOD 1000000007
#define EPS 0.00000001
#define MAXN 1000005
#define ins insert
#define pb push_back
#define mp make_pair
#define sz size
#define rep(i, n) for(int i = 0; i < n; ++i)
#define sd(n) scanf("%d",&n)
#define sll(n) scanf("%I64d",&n)
#define pdn(n) printf("%d\n",n)
#define plln(n) printf("%I64d\n",n)
#define pd(n) printf("%d ",n)
#define nl() printf("\n")
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<vi> vii;
typedef pair<int, int> pii;

int n, k; string s;

int dp[500005][27];

int solve(int idx, char prev) {
	if(idx == 0) {
		if(prev == s[idx])
			return dp[idx][prev-'A'] = 1;
		return dp[idx][prev-'A'] = 0;
	}
	if(dp[idx][prev-'A'] != -1)
		return dp[idx][prev-'A'];
	if(prev == s[idx]) {
		int ans = INF;
		rep(i, k) {
			if(i+'A' != prev)
				ans = min(ans, 1+solve(idx-1, i+'A'));
		}
		return dp[idx][prev-'A'] = ans;
	}
	else {
		int ans = solve(idx-1, s[idx]);
		rep(i, k) {
			if(i+'A' != prev)
				ans = min(ans, 1+solve(idx-1, i+'A'));
		}
		return dp[idx][prev-'A'] = ans;
	}
}

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	rep(i, 500005) {
		rep(j, 27) {
			dp[i][j] = -1;
		}
	}
	cin >> n >> k >> s;
	cout << solve(n-1, '[') << endl;
	return 0;
}