#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pb push_back
#define pii pair<int, int>
#define all(a) a.begin(), a.end()
#define sz(a) ((int)a.size())
const int mod = 1000'000'007, N = 55, K = 22, C = 28;
int add(int x, int y){x+=y; if(x>=mod) x-=mod; return x;}
int mul(int x, int y){return ((ll)x)*y%mod;}

int n,dp[N][N][K+1][C];
string s[N];

int main() {
    ios::sync_with_stdio(false), cin.tie(0);
    cin >> n;
    for(int i=0; i<n; ++i) cin >> s[i];
    for(int i=0; i<n; ++i) while(sz(s[i])<K) s[i]+='a'-1;
    for(int l=0; l<=n; ++l) for(int i=0; i+l<=n; ++i) for(int k=K; k>=0; --k) for(int c=0; c<=27; ++c){
        int j=i+l;
        if(i==j){
            dp[i][j][k][c]=1;
            continue;
        }
        dp[i][j][k][c]=0;
        if(c==0) continue;
        if(k==K){
            dp[i][j][k][c]=j==i+1;
            continue;
        }
        dp[i][j][k][c]=dp[i][j][k][c-1];
        for(int m=j-1; m>=i; --m){
            if(s[m][k]!='?'&&s[m][k]!='a'-1+(c-1)) break;
            if(c==1&&s[m][k]=='?') break;
            dp[i][j][k][c]=add(dp[i][j][k][c],mul(dp[i][m][k][c-1],dp[m][j][k+1][27]));
        }
        //if(dp[i][j][k][c]) cout << i << ' ' << j << ' ' << k << ' ' << c << ' ' << dp[i][j][k][c] << endl;
    }
    cout << dp[0][n][0][27] << "\n";
}