#include <stdio.h>

int main() {
    int n;
    char strings[100][100];
    char prefix[100];
    int i, j;

    /* Read number of strings */
    scanf("%d", &n);

    if (n <= 0) {
        printf("Invalid input\n");
        return 0;
    }

    /* Read strings */
    for (i = 0; i < n; i++) {
        scanf("%99s", strings[i]);
    }

    /* Initialize prefix as first string */
    i = 0;
    while (strings[0][i] != '\0') {
        prefix[i] = strings[0][i];
        i++;
    }
    prefix[i] = '\0';

    /* Compare prefix with remaining strings */
    for (i = 1; i < n; i++) {
        j = 0;
        while (
            prefix[j]     != '\0' && 
            strings[i][j] != '\0' && 
            prefix[j]     == strings[i][j]
        ) {
            j++;
        }
        prefix[j] = '\0';   // cut prefix

        if (prefix[0] == '\0')
            break;          // early exit
    }

    /* Output */
    if (prefix[0] == '\0')
        printf("No common prefix\n");
    else
        printf("Common prefix: %s\n", prefix);

    return 0;
}