#include <bits/stdc++.h>
using namespace std;
const int N=1e3+5,MOD=1e9+7;
int n,k,ans;
int f[N][N];
bool vis[27];
char str[N];
int main(){
scanf("%d%d",&n,&k);
scanf("%s",str+1);
for (register int i=1; i<=n; ++i)
{
f[i][1]=1;
for (register int j=1; j<=k; ++j)
{
memset(vis,false,sizeof(vis));
int now=i-1;
while (now)
{
if (!vis[str[now]-'a'+1]) f[i][j]=(f[i][j]+f[now][j-1])%MOD,vis[str[now]-'a'+1]=true;
now--;
}
}
}
memset(vis,false,sizeof(vis));
for (register int i=n; i>=1; --i) if (!vis[str[i]-'a'+1]) vis[str[i]-'a'+1]=true,ans=(ans+f[i][k])%MOD;
if (k==0) ans=1;
printf("%d\n",ans);
return 0;
} 