#include <bits/stdc++.h>
using namespace std::literals::string_literals;
using i64 = std::int_fast64_t;
using std::cout;
using std::endl;
using std::cin;

template<typename T>
std::vector<T> make_v(size_t a){return std::vector<T>(a);}

template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts){
  return std::vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

int main() {
	int n, k; scanf("%d%d", &n, &k);
	std::vector<std::string> s(n);
	for(int i = 0; i < n; i++) cin >> s[i];
	
	i64 ans = 0;
	std::for_each(begin(s), end(s), [&](auto x) { ans += (x.size() >= k); });
	
	std::vector<int> a(n - 1);
	for(int i = 0; i < n - 1; i++) {
		for(int j = 0; j < std::min(s[i].size(), s[i + 1].size()); j++) {
			if(s[i][j] != s[i + 1][j]) continue;

			a[i] = j + 1;
		}
	}
	
	#ifndef NDEBUG
	for(auto v: a) printf("%d ", v);
	printf("\n");
	#endif
	
	using P = std::pair<int, int>;

	int cur = 0;
	std::priority_queue<P, std::vector<P>, std::greater<P>> qu;
	for(int i = 0; i < a.size(); i++) {
		cur = std::max(cur, i);
		
		while(!qu.empty() and qu.top().second < i) qu.pop();
		while(cur < a.size() and (qu.empty() or qu.top().first >= k) and a[cur] >= k) {
			qu.push({a[cur], cur++});
		}
		
		if(qu.top().first >= k and i < cur) ans += cur - i;
	}	
	printf("%lld\n", ans);
	return 0;
}