#include <bits/stdc++.h>
#include <string>
using namespace std;
int n,m,ans=0;
string s;
struct node
{
	int pos;
	string s;
}a[50001];
int b[50001];
int c[50001];//树状数组闪亮登场 
inline bool operator<(const node &nd,const node &nd2){
	return nd.s<nd2.s;
}

inline const void add(int t)
{
    while(t<=n)
    {
    	c[t]++;
    	t+=t&(-t);
	}
}

inline const int getsum(int t)
{
    int i,sum=0;
    while(t)
    {
    	sum+=c[t];
    	t-=t&(-t);
	}
    return sum;
}
int main()
{
	//freopen("sort2.in","r",stdin);
	//freopen("sort.out","w",stdout);
	int i;
	scanf("%d %d",&n,&m);
	cin>>s;
	for(i=1;i<=n;i++)
		a[i].s=s.substr(i-1,m),a[i].pos=i;
	sort(a+1,a+n+1);
	int p=1;
	b[a[1].pos]=p;
	for(i=2;i<=n;i++)
	{
		if(a[i-1].s!=a[i].s)
			p++;
		b[a[i].pos]=p;
	}
	//for(i=1;i<=n;i++)
		//cout<<b[i]<<" ";
	//cout<<endl;
	for(i=1;i<=n;i++)
    {
		add(b[i]);
		ans+=i-getsum(b[i]);
	}
	cout<<ans;
}