Problem: There are N white balls arranged in a row, numbered 1,2,..,N from left to right. AtCoDeer the deer is thinking of painting some of these balls red and blue, while leaving some of them white.

You are given a string s of length K. AtCoDeer performs the following operation for each i from 1 through K in order:

* The i-th operation: Choose a contiguous segment of balls (possibly empty), and paint these balls red if the i-th character in s is `r`; paint them blue if the character is `b`.



Here, if a ball which is already painted is again painted, the color of the ball will be overwritten. However, due to the properties of dyes, it is not possible to paint a white, unpainted ball directly in blue. That is, when the i-th character in s is `b`, the chosen segment must not contain a white ball.

After all the operations, how many different sequences of colors of the balls are possible? Since the count can be large, find it modulo 10^9+7.

Constraints

* 1 ≤ N ≤ 70
* 1 ≤ K ≤ 70
* |s| = K
* s consists of `r` and `b`.
* N and K are integers.

Input

Input is given from Standard Input in the following format:


N K
s


Output

Print the number of the different possible sequences of colors of the balls after all the operations, modulo 10^9+7.

Examples

Input

2 2
rb


Output

9


Input

5 2
br


Output

16


Input

7 4
rbrb


Output

1569


Input

70 70
bbrbrrbbrrbbbbrbbrbrrbbrrbbrbrrbrbrbbbbrbbrbrrbbrrbbbbrbbrbrrbbrrbbbbr


Output

841634130

Solution (in CPP):

Solution:
#include <bits/stdc++.h>

using namespace std;

#define LOCAL
string to_string(string s) { return '"' + s + '"'; }
string to_string(bool x) { return (x ? "T" : "F"); }
string to_string(const char* s) { return to_string((string)s); }
template <typename A, typename B>
string to_string(pair<A, B> p) { return "(" + to_string(p.first) + ", " + to_string(p.second) + ")"; }
template <typename V>
string to_string(V v) {
    bool f = true; string res = "[";
    for (const auto& x: v) {
        if (!f) res += ", ";
        f = false;
        res += to_string(x); } res += "]";
    return res;
}
void debug() { cerr << endl; }
template <typename H, typename... T>
void debug(H h, T... t) { cerr << " " << to_string(h); debug(t...); }
#ifdef LOCAL
#define dbg(args...) cerr<<"("<<#args<<") =", debug(args)
#else
#define dbg(args...) 2019
#endif

const int MOD = 1000000007; // 998244353; // 998244853;

template <typename T>
struct modular {
    constexpr modular() : val(0){}
    constexpr modular(const modular<T>& _m) : val(_m.val) {}
    template <typename U> constexpr modular(const U& _r = U()) {
        val = -MOD <= _r && _r < MOD ? _r: _r % MOD;
        if (val < 0) { val += MOD; } }
    const T operator()() { return val; }
    template <typename U> explicit operator U() const { return static_cast<U>(val); }
    modular<T>& operator+=(const modular<T>& _m) { if ((val += _m.val) >= MOD) { val -= MOD; } return *this; }
    modular<T>& operator-=(const modular<T>& _m) { if ((val -= _m.val) < 0) { val += MOD; } return *this; }
    modular<T>& operator*=(const modular<T>& _m) { val = modular<T>(static_cast<int64_t>(val) * static_cast<int64_t>(_m.val)).val; return *this; }
    modular<T>& operator/=(const modular<T>& _m) {
        T a = _m.val, b = MOD, u = 0, v = 1;
        while (a != 0) {
            T q = b / a;
            b -= q * a; swap(a, b);
            u -= q * v; swap(u, v); 
        } return *this *= u; }
    modular<T>& operator =(const modular<T>& _m) { val = _m.val; return *this; }
    template <typename U> modular<T>& operator+=(const U& _r) { return *this += modular<T>(_r); }
    template <typename U> modular<T>& operator-=(const U& _r) { return *this -= modular<T>(_r); }
    template <typename U> modular<T>& operator*=(const U& _r) { return *this *= modular<T>(_r); }
    template <typename U> modular<T>& operator/=(const U& _r) { return *this /= modular<T>(_r); } 
    template <typename U> modular<T>& operator =(const U& _r) { val = modular<T>(_r).val; return *this; }
    modular<T> operator-() { return modular<T>(-val); }    
    template <typename U> friend bool operator==(const modular<U>&, const modular<U>&);
    friend std::istream& operator>>(std::istream& os, modular<T>& _m) { os >> _m.val; _m *= 1; return os; }
    friend std::ostream& operator<<(std::ostream& os, const modular<T>& _m) { return os << _m.val; }
    template <typename U>
    modular<T> exp(U e) {
        modular<T> res = 1;
        modular<T> b = val;
        if (e < 0) { b = 1/b; e *= -1; }
        for (; e; e >>= 1) {
            if (e & 1) { res *= b; }
            b *= b;
        } return res; }
private:
    T val;
};
template <typename T> inline modular<T> operator+(const modular<T>& _lhs, const modular<T>& _rhs) { return modular<T>(_lhs) += _rhs; }
template <typename T, typename U> inline modular<T> operator+(const modular<T>& _lhs, const U& _rhs) { return modular<T>(_lhs) += _rhs; }
template <typename T, typename U> inline modular<T> operator+(const U& _lhs, const modular<T>& _rhs) { return modular<T>(_lhs) += _rhs; }
template <typename T> inline modular<T> operator-(const modular<T>& _lhs, const modular<T>& _rhs) { return modular<T>(_lhs) -= _rhs; }
template <typename T, typename U> inline modular<T> operator-(const modular<T>& _lhs, const U& _rhs) { return modular<T>(_lhs) -= _rhs; }
template <typename T, typename U> inline modular<T> operator-(const U& _lhs, const modular<T>& _rhs) { return modular<T>(_lhs) -= _rhs; }
template <typename T> inline modular<T> operator*(const modular<T>& _lhs, const modular<T>& _rhs) { return modular<T>(_lhs) *= _rhs; }
template <typename T, typename U> inline modular<T> operator*(const modular<T>& _lhs, const U& _rhs) { return modular<T>(_lhs) *= _rhs; }
template <typename T, typename U> inline modular<T> operator*(const U& _lhs, const modular<T>& _rhs) { return modular<T>(_lhs) *= _rhs; }
template <typename T> inline modular<T> operator/(const modular<T>& _lhs, const modular<T>& _rhs) { return modular<T>(_lhs) /= _rhs; }
template <typename T, typename U> inline modular<T> operator/(const modular<T>& _lhs, const U& _rhs) { return modular<T>(_lhs) /= _rhs; }
template <typename T, typename U> inline modular<T> operator/(const U& _lhs, const modular<T>& _rhs) { return modular<T>(_lhs) /= _rhs; }
template <typename T> inline bool operator==(const modular<T>& _lhs, const modular<T>& _rhs) { return _lhs.val == _rhs.val; }
template <typename T, typename U> inline bool operator==(const modular<T>& _lhs, const U& _rhs) { return _lhs == modular<T>(_rhs); }
template <typename T, typename U> inline bool operator==(const U& _lhs, const modular<T>& _rhs) { return modular<T>(_lhs) == _rhs; }
template <typename T> inline bool operator!=(const modular<T>& _lhs, const modular<T>& _rhs) { return !(_lhs == _rhs); }
template <typename T, typename U> inline bool operator!=(const modular<T>& _lhs, const U& _rhs) { return !(_lhs == _rhs); }
template <typename T, typename U> inline bool operator!=(const U& _lhs, const modular<T>& _rhs) { return !(_lhs == _rhs); }
typedef modular<int> mint;

struct Comb {
    int N;
    vector<mint> fac, ifac, inv;

    Comb(int _n=1) {
        resize(2);
        fac[0] = fac[1] = ifac[0] = ifac[1] = inv[1] = 1;
        N = 1; grow(_n);
    }

    void resize(int _s) {
        fac.resize(_s); ifac.resize(_s); inv.resize(_s);
    }
    bool grow(int _n) {
        if (_n <= N) return false;
        swap(_n, N); resize(N+1);
        for (int i = _n+1; i <= N; i++) {
            fac[i] = fac[i-1] * i;
            inv[i] = (-MOD/i) * inv[MOD%i];
            ifac[i] = ifac[i-1] * inv[i];
        }
        return true;
    }
    // perm
    mint a(int n, int k) {
        if (n < k || k < 0) return 0;
        if (n > N) grow(n);
        return fac[n] * ifac[n-k];
    }
    // choose
    mint c(int n, int k) {
        if (n < k || k < 0) return 0;
        if (n > N) grow(n);
        return fac[n] * ifac[n-k] * ifac[k];
    }
    // sum x = s, x > 0
    mint p(int s, int k) {
        return c(s-1, k-1);
    }
    // sum x = s, x >= 0
    mint z(int s, int k) {
        return p(s+k, k);
    }
    // \E Y-X >= m
    mint hit(int n, int k, int m=1) {
        if (m <= k-n) return c(n+k, n);
        return c(n+k, k-m);
    }
    // catalan trapezoids C_m(n, k)
    // \A Y-X < m,  =c(n+k,n) - hit
    mint cat(int n, int k, int m=1) {
        if (m > k) return c(n+k, n);
        if (m <= k-n) return 0;
        return c(n+k,n) - c(n+k, k-m);
    }
} c(150);

mint res;
int n,k;
string s;
bool trk[77];
int pos[77];
int cnt[77];

bool check(const vector<int>& ip, int sum) {
    memset(trk, 0, sizeof trk);
    memset(pos, -1, sizeof pos);
    memset(cnt, 0, sizeof cnt);
    int m = ip.size();
    for (int i = m-1; i >= 0; i--) {
        for (int j = 0; j < k; j++) {
            if (s[j]=='r' && !trk[j]) {
                pos[i] = j; trk[j] = true;
                cnt[i]++;
                break;
            }
        }
        if (pos[i] == -1) return false;
    }
    for (int i = m-1; i >= 0; i--) {
        if (ip[i] == 1) continue;
        for (int j = pos[i]+1; j < k; j++) {
            if (s[j]=='b' && !trk[j]) {
                cnt[i]++;
                pos[i]=j; trk[j] = true;
                break;
            }
        }
        if (cnt[i] != 2) return false;
    }
    for (int i = m-1; i >= 0; i--) {
        for (int j = pos[i]+1; ip[i] > cnt[i] && j < k; j++) {
            if (!trk[j]) {
                cnt[i]++;
                pos[i] = j; trk[j] = true;
            }
        }
        //cout << i << ','<<cnt[i]<<'\n';
        if (ip[i] != cnt[i]) return false;
    }
    
    mint coeff = c.fac[m];
    int las = 0, len = 0;
    for (const int& x: ip) {
        if (x == las) len++;
        else {
            coeff *= c.ifac[len];
            len = 1;
            las = x;
        }
    }coeff *= c.ifac[len];

    int zero = 0;
    zero += 2;
    for (const int& x: ip) {
        if (x > 1) zero+=2;
    }

    //dbg(ip, sum);
    if (sum == 0) {
        res += 1; return true;
    }
    //cout << coeff << '\n';
    res += c.p(n+zero, sum+zero) * coeff;
    //cout << res << "\n";
    return true;
}

void dfs(vector<int>& ip, int sum) {
    check(ip, sum);
    //dbg(ip);
    int sta = ip.empty()? 1 : ip.back();
    for (int i = sta; i <= k; i++) {//lim not n,
        int nex_sum = sum + max(2*i-3, 1) + (ip.empty()?0:1);
        if (nex_sum > n) break;
        ip.emplace_back(i);
        dfs(ip, nex_sum);
        ip.pop_back();
    }
}

void solve() {
    cin >> n >> k;
    cin >> s;
    vector<int> ip;
    dfs(ip, 0);
    cout << res;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    solve();
    cout << endl;
}