#include <iostream>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <string>
#include <algorithm>
#include <vector>
#include <stack>
#include <deque>
#include <queue>
#include <map>
#include <set>
#include <climits>

using namespace std;

#define N (int) 1e6 + 7
#define INF (int) (2e9)
#define EPS 1e-12
#define MOD (int) (1e9 + 7)
#define ord1(x) ((int) (x) - (int) '0')
#define ordA(x) ((int) (x) - (int) 'a' + 1)
#define Freopen freopen("input.txt", "r", stdin); \
                freopen("output.txt", "w", stdout);
#define forn(it, left, right) for (int it = left; it <= (int) right; it++)
#define mp make_pair
#define pb push_back
#define ll long long

char check (int, int);

int L, n, k;
ll ans;

char s[N];
int p[N][37];
string f = "0123456789abcdefghijklmnopqrstuvwxyz";

int main()
{
    Freopen
    cin >> n >> k;
    k--;
    gets(s);
    gets(s);

    for (int i = 0; i < 36; i++)
    {
        if (s[0] == f[i])
            p[0][i] = 1;
        else
            p[0][i] = 0;
        for (int j = 1; j < n; j++)
            if (s[j] == f[i])
                p[j][i] = p[j - 1][i] + 1;
            else
                p[j][i] = p[j - 1][i];
    }

   /* for (int i = 0; i < 36; i++)
    {
        for (int j = 0; j < n; j++)
            cout << p[j][i] << ' ';
        cout << endl;
    }*/

    L = 0;
    for (int i = 0; i < n; i++)
        for (int j = L; j <= i; j++)
            if (check(j, i))
            {
            //    cout<<j<<' '<<i<<endl;
                ans += i - j + 1;
                L = j;
                break;
            }

    cout << ans;
    return 0;
}

char check (int l, int r)
{
    int cnt = 0;
    for (int i = 0; i < 36; i++)
        if (p[r][i] - p[l - 1][i] != 0)
        {
            cnt = 0;
            if (l == 0)
                cnt += p[r][i] - 1;
            else
                cnt += p[r][i] - p[l - 1][i] - 1;
            if (cnt > k)
                return 0;
        }
    return 1;
}