C.cpp

//#pragma GCC optimize("Ofast")
//#pragma GCC optimize ("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include "bits/stdc++.h"
#include<ext/pb_ds/assoc_container.hpp>
#include <random>
#include<ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;
template<class T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag,
        tree_order_statistics_node_update>;
using namespace std;
#define ll long long
#define Silver_is_better_than_Gold ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define all(x) x.begin(),x.end()
#define allr(x) x.rbegin(),x.rend()
#define pb push_back
#define endl '\n'
#define test   cout<<" We Droped up Here Boss _________________ "<<endl
#define pii pair<int,int>
#define vi vector<int>
#define vl vector<ll>
#define vvi vector<vector<int>>
#define vvl vector<vector<ll>>
#define no {cout<<"NO"<<endl;}
#define yes {cout<<"YES"<<endl;}
const int mod = 1e9 + 7;
#define sz(x) ((int) (x).size())
#define dups(divs)     divs.erase(unique(divs.begin(), divs.end()), divs.end());
#define sum(v) accumulate(v.begin(),v.end(),0ll)
#define T int XXX;cin>>XXX;while(XXX--)
#define F first
#define S second
#define ld long double
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};

template<class A>
istream &operator>>(istream &cin, pair<A, A> p) {
    return cin >> p.first >> p.second;
}

template<class A>
istream &operator>>(istream &cin, vector<A> &p) {
    for (int i = 0; i < sz(p) - 1; i++)
        cin >> p[i];
    return cin >> p.back();
}

template<class A>
ostream &operator<<(ostream &cout, pair<A, A> const &v) {
    return cout << v.first << ' ' << v.second << endl;
}

template<class A>
ostream &operator<<(ostream &cout, vector<A> const &v) {
    for (auto ele: v) { cout << ele << " "; }
    cout << endl;
    return cout;
}

ll fastpow(ll base, ll power, ll M) {
    if (power == 1) return base;
    ll val = fastpow(base, power / 2, M);
    return (power % 2 == 0) ? (val * val) % M : (((val * val) % M) * base) % M;
}

vector<char> adj[1050];
map<char, int> vis, recStack;
vector<char> answer;

bool dfs(char father) {
    vis[father] = 1;
    recStack[father] = 1;
    for (auto i: adj[father]) {
        if (!vis[i] and dfs(i)) {
            return true;
        }
        if (recStack[i]) {
            return true;
        }
    }
    recStack[father] = false;
    answer.push_back(father);
    return false;
}

signed main() {
    int n;
    cin >> n;
    vector<string> v(n);
    for (auto &i: v)cin >> i;
    for (int i = 0; i < n - 1; i++) {
        int N = min(sz(v[i]), sz(v[i + 1]));
        bool x = 0;
        for (int j = 0; j < N; j++) {
            if (v[i + 1][j] != v[i][j]) {
                x = 1;
                adj[v[i + 1][j]].push_back(v[i][j]);
                break;
            }
        }
        if (!x and sz(v[i]) > sz(v[i + 1])) {
            cout << "Impossible";
            exit(0);
        }
    }
    for (char i = 'a'; i <= 'z'; i++)
        if (!vis[i] and dfs(i)) {
            cout << "Impossible";
            exit(0);
        }
    for (auto i: answer)cout << i;
}