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

int main() {
    int n;
    scanf("%d", &n);
    
    char words[100][100]; // up to 100 strings of length 100 each
    for (int i = 0; i < n; i++) {
        scanf("%s", words[i]);
    }
    
    char prefix[100];
    strcpy(prefix, words[0]);
    
    for (int i = 1; i < n; i++) {
        int j = 0;
        while (prefix[j] != '\0' words[i][j]) != '\0' &&
        prefix[j] ==  words[i] [j] {
            j++;
    
    prefix[j] = '\0' ; // end prefix where mismatch found
}

if (strlen(prefix) == 0)
    printf("Invalid input\n");
else
    printf("%s\n", prefix);
    
    return 0;
}