#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define X first
#define Y second
#define pb push_back
#define ALL(v) v.begin(), v.end()
#define SZ(a) ((int)a.size())

const int MOD = 1e9 + 7;

void add(int &x, int val) {
    x += val;
    if (x >= MOD) x -= MOD;
}

int fac[30];
int dp[30][700];

void run_dp(vector<int> cost) {
    memset(dp, 0, sizeof dp);
    dp[0][0] = 1;
    int sum = 0;
    for (int v : cost) {
        sum += v;
        for (int i = SZ(cost); i >= 1; --i)
            for (int j = v; j <= sum; ++j)
                add(dp[i][j], dp[i - 1][j - v]);
    }
}

int main() {
    ios::sync_with_stdio(0), cin.tie(0);
    fac[0] = 1;
    for (int i = 1; i < 30; ++i)
        fac[i] = (ll)fac[i - 1] * i % MOD;
    int n, kth, ans = 0;
    string s;
    cin >> n >> kth >> s;
    for (int i = 0; i + SZ(s) <= n; ++i) {
        int zero = s[0] - 'a' + 1, one = n - zero, pre = 0;
        for (int j = 0; j < SZ(s); ++j) {
            if (s[j] <= s[0]) --zero;
            else --one;
            if (s[j] < s[0]) pre += n - (i + j);
        }
        int base = SZ(s) + pre;
        if (base > kth) continue;
        vector<int> cost;
        for (int k = 0; k < i; ++k)
            cost.pb(n - k);
        for (int k = i + SZ(s); k < n; ++k)
            cost.pb(n - k);
        run_dp(cost);
        add(ans, (ll)dp[zero][kth - base] * fac[zero] % MOD * fac[one] % MOD);
    }
    cout << ans << "\n";
}