#include<bits/stdc++.h>
using namespace std;
#define maxn 1000010
#define ll long long
const int mod=1e9+7;
const int c=26;
const int p=101;
const int Mod=1e9+7;
int n,k;
char s[maxn];
ll pw(ll a,int n)
{
        ll res=1;
        while(n)
        {
                if(n&1)res=res*a%mod;
                a=a*a%mod,n>>=1;
        }
        return res;
}
ll INV(ll a){return pw(a,mod-2);}
int main()
{
        scanf("%d%d",&n,&k);
        scanf("%s",s);
        ll pre=0,suf=0;
        ll ans=0;
        ll pp=1;
        for(int i=0;i<n;i++)
        {
                pre=(pre*p+(s[i]-'a'))%Mod,suf=(suf+pp*(s[n-i-1]-'a'))%Mod;
                pp=pp*p%Mod;
                if(pre==suf && 2*n-(i+1)<=k)ans++;
        }
        if(k>=2*n)
        {
                /*
                   int m=k-2*n;
                   ll tmp=pw(c,m+1);
                   tmp=(tmp-1+mod)%mod;
                   tmp=tmp*INV(c-1)%mod;
                   ans=(ans+tmp)%mod;
                   */
                int m=k-2*n;
                ll cur=1;
                for(int i=0;i<=m;i++)
                {
                        ans=(ans+cur)%mod;
                        cur=cur*c%mod;
                }
        }
        printf("%lld\n",ans);
        return 0;
}