#include <stdio.h>
#include <string.h>
#include <stdlib.h
#define MAX_N 100
#define MAX_LEN 100

void longestCommonPrefix(char arr[][MAX_LEN + 1], int n) {
    char prefix[MAX_LEN + 1];
    strcpy(prefix, arr[0]); 
    for (int i = 1; i < n; i++) {
        int j = 0;
        while (prefix[j] && arr[i][j] && prefix[j] == arr[i][j]) {
            j++;
        }
        prefix[j] = '\0'; 
        if (prefix[0] == '\0') {
            printf("Invalid input\n");
            return;
        }
    }
    printf("%s\n", prefix);
}

int main() {
    int n;
    scanf("%d", &n);
    char arr[MAX_N][MAX_LEN + 1];
    for (int i = 0; i < n; i++) {
        scanf("%s", arr[i]);
    }
    longestCommonPrefix(arr, n);

    return 0;
}