#include <stdio.h>
#include <string.h>
int main() {
    int n;
    scanf("%d", &n);
    if (n <= 0) {
        printf("Invalid input");
        return 0;
    }
    char arr[n][101];
    for (int i = 0; i < n; i++)
    scanf("%s", arr[o]);
    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 (j == 0) {
            printf("Invalid input");
            return 0;
        }
    }
    printf("%s", prefix);
    return 0;
}