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

int main(){
    int n;
    scanf("%d", &n);
    
    char words[n][101];
    for (int i = 0; i < n; i++){
        scanf("%s", words[i]);
    }
    char prefix[101];
    strcpy(prefix, words[0]);
    
    for (int i = 1; i < n; i++) {
        int j = 0;
        while (prefix[j] && words[i][j] && prefix[j] == words[i][j])
            j++;
        }
        prefix[j] = '\0';
        
        if (strlen(prefix) == 0) {
            printf("Invalid input");
            return 0;
        }
    } 
    
    printf("%s", prefix[j]);
    return 0;
}