#include <bits/stdc++.h>
using namespace std;
 
int main() {
    int n, m;
    cin >> n >> m;
    int a[n+1];
    int ans = 0, dem = 0;
    map<int, int> p;
    for (int i = 1; i <= n; i++) {
        string b;
        cin >> b;
        a[i]= b.size();
        dem++;
        if (dem > m+1) {
            p[a[i-m-1]]--;
            dem--;
        }
 
        ans -= (p[a[i]] * (p[a[i]] - 1)) / 2;
        p[a[i]]++;
        ans += (p[a[i]] * (p[a[i]] - 1)) / 2;
    }
    cout << ans << endl;
    return 0;
}
 

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