#include <stdio.h>
#include <string.h>
#define MAX_STRINGS 100
#define MAX_LENGTH 101

int main() 
{
    int n;
    char strings[MAX_STRINGS][MAX_LENGTH];
    if (scanf("%d", &n) != 1 || n < 1 || n > 100)
    {
        printf("Invalid input\n");
        return 0;
    }

    for (int i = 0; i < n; i++)
    {
        if (scanf("%s", strings[i]) != 1)
        {
            printf("Invalid input\n");
            return 0;
        }
    }
    char *first = strings[0];
    int prefixLen = strlen(first);

    for (int i = 1; i < n; i++)
    {
        int j = 0;
        while (j < prefixLen && strings[i][j] == first[j]) 
        {
            j++;
        }
        prefixLen = j;
    }

    if (prefixLen == 0) 
    {
        printf("Invalid input\n"); 
    }
    else
    {
        for (int i = 0; i < prefixLen; i++) 
            printf("%c", first[i]);
        }
        printf("\n");
    }

    return 0;
}