#include <stdio.h>

int main(void) {
	char c1[100],c2[100];
	scanf("%s%s",c1,c2);
	for (int i = 0;i < 100;i ++){
		if (c1[i] != c2[i]){
			if (i == 0){
				printf("No common prefix");
				break;
			}
			else {
				break;
			}
		}
		printf("%c",c1[i]);
	}
	return 0;
}