#include<bits/stdc++.h>
#define maxn 75
using namespace std;
const int mod = 1e9+7;
int n,k,m,a[maxn],x[maxn],C[maxn*2][maxn*2],ans=1;
char s[maxn];
void dfs(int d,int l,int r,bool chose){
	if(chose){
		int R=1,B=1,S=0;
		for(int i=1;i<=k;i++){
			if(s[i]=='r') {if(R<=m) R++; else if(S) S--;}
			if(s[i]=='b') {if(B<R&&x[B]) S+=x[B++]-1; else if(S) S--;}
		}
		if(R>m&&(B>m||!x[B])&&!S){
			int ret=C[n-l+r][r];
			for(int i=d+1;i<=x[1];i++) S+=a[i],ret=1ll*ret*C[S][a[i]]%mod;
			ans=(ans+ret)%mod;
		}
	}
	if(d<0) return;
	int tm=m,tl=(d?2*d-1:1),tr=2*d+1;
	for(int i=0;l<=n;x[++m]=d,l+=tl+(l!=0),r+=tr+1,i++) 
		a[d]=i,dfs(d-1,l,r,i);
	m=tm;
}
int main()
{
	scanf("%d%d%s",&n,&k,s+1);
	for(int i=C[0][0]=1;i<150;i++) for(int j=C[i][0]=1;j<=i;j++) C[i][j]=(C[i-1][j-1]+C[i-1][j])%mod;
	dfs(n,0,0,0);
	printf("%d\n",ans);
}