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

//字符串匹配
int StringMatch(char *TStr,char *MStr)
{
	char t[100000];
	int i = 0,j,k = 0,l,flag = 0;
	while(TStr[i] != '\0')
	{
		j = 0;
		l = i;
		while(TStr[l] == MStr[j]) l++,j++;
		if(MStr[j] == '\0') //匹配
			flag  =1,i = l;
		else	//不匹配
		{
			if(i == l)
				t[k++] = TStr[i++];
			else
				while(i < l)
				t[k++] = TStr[i++];
		}	
	}
	if(flag)
	{
		t[k] = '\0';
		strcpy(TStr,t);
		return 1;
	}
	else return 0;
}

int main()
{
	char Text_Str[100000],Mode_Str[100000];
	scanf("%s %s",Mode_Str,Text_Str);
	if(StringMatch(Text_Str,Mode_Str))
		puts(Text_Str);
	else printf("-1");
	
	return 0;
}