// Title:  字符串
// Source: SNOI2020
// Author: Jerrywang
#include <cstdio>
#include <iostream>
#include <vector>
#define ll long long
#define rep(i, s, t) for(int i=s; i<=t; ++i)
#define debug(x) cerr<<#x<<":"<<x<<endl;
const int N=300010;
using namespace std;

char s[N]; int n, m, k, rnk[N+N], last[N+N], sa[N], tmp[N], cnt[N], ht[N];
void init()
{
    rep(i, 1, n) rnk[i]=s[i];
    rep(i, n+1, n+n) last[i]=rnk[i]=0;
    int mx=127; rep(i, 1, n) sa[i]=i;
    for(int len=2; len/2<=n; len<<=1)
    {
        rep(i, 1, n) last[i]=rnk[i];
        rep(i, 0, mx) cnt[i]=0;
        rep(i, 1, n) cnt[last[i+len/2]]++;
        rep(i, 1, mx) cnt[i]+=cnt[i-1];
        rep(i, 1, n) tmp[cnt[last[i+len/2]]--]=i;
        rep(i, 0, mx) cnt[i]=0;
        rep(i, 1, n) cnt[last[i]]++;
        rep(i, 1, mx) cnt[i]+=cnt[i-1];
        for(int i=n; i; i--)
        {
            int x=tmp[i];
            sa[cnt[last[x]]--]=x;
        }
        mx=1; rnk[sa[1]]=1;
        rep(i, 2, n)
        {
            if(!(last[sa[i-1]]==last[sa[i]] && last[sa[i-1]+len/2]==last[sa[i]+len/2]))
                mx++;
            rnk[sa[i]]=mx;
        }
        if(mx==n) break;
    }
    int h=0;
    rep(i, 1, n)
    {
        if(rnk[i]==1) continue;
        if(h) h--;
        int j=sa[rnk[i]-1];
        while(s[i+h]==s[j+h] && i+h<=n && j+h<=n) h++;
        ht[rnk[i]]=h;
    }
}

int fa[N], sz[N], id[N]; vector<int> vec[N];
int root(int u) {return fa[u]==u?u:fa[u]=root(fa[u]);}
ll res;
void merge(int u, int v, int i)
{
    u=root(u), v=root(v);
    if(u==v) return;
    if(id[u]+id[v]==3)
    {
        if(sz[u]<sz[v]) swap(u, v);
        res+=(ll)i*sz[v];
        sz[u]-=sz[v], fa[v]=u;
    }
    else
    {
        sz[u]+=sz[v], fa[v]=u;
        id[u]=max(id[u], id[v]);
    }
}

int main()
{
#ifdef Jerrywang
    freopen("in.txt", "r", stdin);
#endif
    scanf("%d%d", &m, &k); n=m+m;
    scanf("%s", s+1), scanf("%s", s+m+1);
    init();
    rep(i, 1, n)
    {
        fa[i]=i;
        if(sa[i]+k-1<=m) id[i]=1, sz[i]=1;
        else if(sa[i]>m && sa[i]+k-1<=n) id[i]=2, sz[i]=1;
    }
    rep(i, 2, n) vec[ht[i]].push_back(i);
    for(int i=n; i; i--)
    {
        rep(j, 0, (int)vec[i].size()-1)
            merge(vec[i][j], vec[i][j]-1, min(i, k));
    }
    printf("%lld", (ll)k*(m-k+1)-res);

    return 0;
}