This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
#define ll long long
#define pii pair<int,int>
#define pll pair<ll,ll>
#define rep(i,m,n) for(int i=(m); i<=(n); i++)
#define reb(i,m,n) for(int i=(m); i>=(n); i--)
#define iter(id, v) for(auto id : v)
#define fs first
#define se second
#define MP make_pair
#define pb push_back
#define bit(msk, i) ((msk >> i) & 1)
#define SZ(v) (ll)v.size()
#define ALL(v) v.begin(),v.end()
using namespace std;
mt19937_64 rd(chrono :: steady_clock :: now ().time_since_epoch().count());
ll Rand (ll l, ll r) { return uniform_int_distribution<ll> (l, r) (rd); }
const int N = 50 + 7;
const int M = 2e3 + 7;
const int Mod = 1e9 + 7;
const int INF = 1e9;
const ll BASE = 137;
const int szBL = 350;
int n, K;
string a[M];
map<string, int> pdp, dp;
void solution() {
cin >> n >> K;
rep (i, 1, n) {
rep (j, 1, K) cin >> a[j];
rep (j, 1, K) {
string &s = a[j];
if (i == 1) dp[s]++;
else {
string s1, s2;
s1 = s.substr(1, SZ(s) - 1);
s2 = s.substr(0, SZ(s) - 1);
if (s1 != s2) dp[s] = ((dp[s] + pdp[s1]) % Mod + pdp[s2]) % Mod;
else dp[s] = (dp[s] + pdp[s1]) % Mod;
}
}
rep (j, 1, K) {
pdp[a[j]] = dp[a[j]];
// cout << a[j] <<": "<<pdp[a[j]] <<"\n";
}
dp.clear();
}
sort (a + 1, a + 1 + n);
n = unique(a + 1, a + 1 + n) - a - 1;
int res = 0;
rep (i, 1, K) res = (res + pdp[a[i]]) % Mod;
cout << res <<"\n";
}
#define file(name) freopen(name".inp","r",stdin); \
freopen(name".out","w",stdout);
int main () {
// file("c");
ios_base :: sync_with_stdio(false); cin.tie(0); cout.tie(0);
int num_Test = 1;
// cin >> num_Test;
while (num_Test--)
solution();
}
/*
no bug +7
5 3 1
1 1 1 2 3
2 3 5
1 2
2 3
2 4
1 5
2 3
*/
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