#include <iostream>
#include<vector>
#include<string>
using namespace std;

int main() {
   ios::sync_with_stdio(false);
   cin.tie(nullptr);
   cout.tie(nullptr);
   int n;
   cin>>n;
   vector<string> v;
   v.reserve(n);
   for(int i=0;i<n;i++){
      string s;
      cin>>s;
      v.emplace_back(s);
   }
    char c;
    for(c='a';c<='z';c++){
       bool found=true;
       for(const string& s:v){
            if(s.find(c)==string::npos){
                found=false;
                break;
            }
       }
       if(found) break;
    }
    cout<<c;
    return 0;
}