#include<cstdio>
#include<stack>
using namespace std;
int main(){
// freopen("C:\\Users\\Frank Wang\\Desktop\\in.txt","r",stdin);
stack<char> s1,s2,s3;
int n;
scanf("%d",&n);
getchar();
char c;
while((c=getchar())!='\n'){
s1.push(c);
}
for(int i=1;i<n;i++){
while((c=getchar())!='\n'){
s2.push(c);
}
while(!s1.empty()&&!s2.empty()&&s1.top()==s2.top()){
s3.push(s1.top());
s1.pop();
s2.pop();
}
while(!s1.empty()) s1.pop();
while(!s3.empty()){
s1.push(s3.top());
s3.pop();
}
while(!s2.empty()) s2.pop();
}
if(s1.empty()){
printf("nai\n");
}
else{
while(!s1.empty()){
s2.push(s1.top());
s1.pop();
}
if(s2.top()==' ') s2.pop();
while(!s2.empty()){
printf("%c",s2.top());
s2.pop();
}
putchar('\n');
}
}