KOI06: Cecil

#include <bits/stdc++.h>

using namespace std;
const int N = 1e2 + 10;
const int AZ = 26;
string str[N];
int len[N], indeg[N];
vector <int> g[AZ + 10];
bool inpath[AZ + 10];

int to_int(char a){
    return a - 'a' + 1;
}

char to_char(int a){
    return a + 'a' - 1;
}

int main(){

    int n;
    scanf("%d", &n);

    for(int i=1;i<=n;i++){
        cin >> str[i];
        len[i] = str[i].size();
        if(i == 1) continue;
        int pre = 0, cur = 0;
        while(pre < len[i - 1] and cur < len[i] and str[i - 1][pre] == str[i][cur]){
            pre ++, cur ++;
        }
        if(pre < len[i - 1] and cur < len[i]) g[to_int(str[i - 1][pre])].push_back(to_int(str[i][cur]));
    }

    for(int u=1;u<=AZ;u++){
        if(g[u].empty()) continue;
        for(auto v: g[u]){
            indeg[v] ++;
            inpath[v] = true;
        }
    }

    priority_queue <int, vector <int>, greater <int>> q;
    for(int i=1;i<=AZ;i++)
        if(indeg[i] == 0) q.push(i);
    vector <int> seq;
    while(!q.empty()){
        int u = q.top(); q.pop();
        seq.push_back(u);
        for(auto v: g[u]){
            indeg[v] --;
            if(indeg[v] == 0) q.push(v);
        }
    }

    if(seq.size() < 26) printf("-1");
    else{
        for(auto s: seq) printf("%c", to_char(s));
    }

    return 0;
}