#include<stdio.h>
#include<string.h>

int main()
{
    int n;
    scanf("%d",&n);
    char s[100][101];
    for(int i = 0; i < n; i++)
    { scanf("%d",s[i]); }
    char lcp[101];
    strcpy(lcp, s[0]);
    for(int i = 1; i < n; i++)
    { int j = 0
    while(lcp[j] && s[i][j] && lcp[j] == s[i][j]) j++
    lcp[j]='';
    if(j==0)
    { printf("Invalid input");
    return 0; }
    }
    printf("%s",lcp);
    return 0;
}