#include<<stdio.h>
#include<string.h>
int main()
{
    int N;
    scanf("%d", &N);
    if (N <= 0) {
        printf("Invalid input");
        return 0;
    }
    char arr[100][101];
    for (int i = 0; i < N; i++) {
        scanf("%s", arr[i]);
    }
    char result[101];
    int index = 0;
    while (1) {
        char current = arr[0][index];
        if (current == '\0')
        break;
        for (int i = 1; i < N; i++) {
            if (arr[i][index] != current) {
                if (index == 0)
                printf("Invalid input");
                else
                printf("%.*s", index, arr[0]);
                return 0;
            }
        }
        index++;
    }
    if (index == 0)
    printf("Invalid input");
    else
    printf("%.*s", index, arr[0]);
    return 0; 
}