#include<bits/stdc++.h>

using namespace std;
#define LL long long
const int N = 1e6 + 10;
const int INF = 0x3f3f3f3f;
int n,k,t[60];
char s[N];
int main(){
	cin >> n >> k;
	scanf("%s",s+1);
	for(int i = 1; i <= n;i++)
		if(isupper(s[i]))
			t[s[i] - 'A']++;
		else
			t[s[i] - 'a' + 26]++;
	sort(t,t+52);
	int now = 0;
	while(!t[now])
		now++;
	t[51] += k;
	while(k > 0)
	{
		if(k > t[now])
			k -= t[now] , t[now] = 0;
		else
			t[now] -= k,k = 0;
		now++;
	}
	long long ans = 0;
	for(int i = 0;i < 52;i++)
		ans += 1ll * t[i] * (t[i] - 1) / 2;
	cout << ans << endl;
	return 0;
}