#include<ctime>
#include<stdio.h>
#include<string>
#include<algorithm>
#include<iostream>
#include<vector>
#include<set>
#include<map>
#include<queue>
#include<list>
#include<memory.h>
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int inf = 1e9+7;
const ld eps = 1e-16;
int n;
char a[55][22], len[55];
int f[22][55][55][30];
bool marked[22][55][55][30];
int maxLen[55][55];
int qc[22][55][55];

int rec(int c, int l, int r, char maxQ){
	if(r < l)
		return 1;
	if(c >= maxLen[l][r])
		return 0;

	if(marked[c][l][r][maxQ])
		return f[c][l][r][maxQ];
	marked[c][l][r][maxQ] = true;

	if(l == r){
		if(c > len[l]){
			return f[c][l][r][maxQ] = 0;
		}else{
			ll q = 1;
			for(int i = c; i < len[l]; i++){
				if(a[l][i] == '?'){
					q = (q * maxQ) % inf;
					maxQ = 26;
				}else if(a[l][i] - 'a' < maxQ){
					maxQ = 26;
				}
			}
			return f[c][l][r][maxQ] = q;
		}
	}

	int res = 0;
	int minQ = -1;

	for(int i = l; i <= r; i++){
		char w = qc[c][i][r];
		if(w == 1)
			continue;

		if(w != '?' && w - 'a' <= minQ)
			continue;

		if(a[i][c] != '?' && a[i][c] - 'a' > minQ){
			minQ = a[i][c] - 'a';
		}

		if(w == '?'){
			for(int q = minQ + 1; q < maxQ; q++){
				res += ((ll)rec(c + 1, i, r, 26) * rec(c, l, i - 1, q)) % inf;
				if(res > inf)
					res -= inf;
			}
		}else if(w - 'a' < maxQ){
			res += ((ll)rec(c + 1, i, r, 26) * rec(c, l, i - 1, w - 'a')) % inf;
			if(res > inf)
				res -= inf;
		}
	}

	return f[c][l][r][maxQ] = res;
}

int main(){
#ifdef _BSUIR2JUDGE_
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#else
	//freopen("sum.in", "r", stdin);
	//freopen("sum.out", "w", stdout);
#endif

	memset(a, 0, sizeof(a));

	scanf("%d\n", &n);
	for(int i = 0; i < n; i++){
		scanf("%s\n", &a[i][0]);
		len[i] = strlen(a[i]);
	}

	for(int i = 0; i < n; i++){
		int ml = 0;

		for(int j = i; j < n; j++){
			ml = max(ml, (int)len[j]);
			maxLen[i][j] = ml;
		}
	}

	for(int k = 0; k < 22; k++){
		for(int i = 0; i < n; i++){
			char w = '?';

			for(int j = i; j < n; j++){
				if(w != 1){
					if(a[j][k] != '?' && w != '?' && a[j][k] != w){
						w = 1;
					}else if(a[j][k] != '?'){
						w = a[j][k];
					}
				}

				qc[k][i][j] = w;
			}
		}
	}

	cout << rec(0, 0, n - 1, 26);

	return 0;
}