#include <bits/stdc++.h>
using namespace std;
int main() {
    int n; cin>>n;
    vector<string> words(n);
    set<char> chars;
    for (int i=0;i<n;i++) { cin>>words[i]; for (char c:words[i]) chars.insert(c); }
    map<char,set<char>> adj;
    map<char,int> ind;
    for (char c:chars) ind[c]=0;
    bool ok=true;
    for (int i=0;i<n-1&&ok;i++) {
        string &w1=words[i],&w2=words[i+1];
        int len=min(w1.size(),w2.size());
        if (w1.size()>w2.size()&&w1.substr(0,len)==w2.substr(0,len)) { ok=false; break; }
        for (int j=0;j<(int)len;j++) if (w1[j]!=w2[j]) {
            if (!adj[w1[j]].count(w2[j])) { adj[w1[j]].insert(w2[j]); ind[w2[j]]++; }
            break;
        }
    }
    if (!ok) { cout<<"INVALID\n"; return 0; }
    priority_queue<char,vector<char>,greater<char>> pq;
    for (auto& p:ind) if (p.second==0) pq.push(p.first);
    string res;
    while (!pq.empty()) {
        char u=pq.top(); pq.pop(); res+=u;
        for (char v:adj[u]) if (--ind[v]==0) pq.push(v);
    }
    cout<<(res.size()<chars.size()?"INVALID":res)<<"\n";
}