代码如下

#include<cstdio>
int i;
char max(char ,char );
int main()
{
	int n,f;
	scanf("%d",&n);
	char a[110],b[110],c[110],a1,b1,d,t;
	gets(a);
	gets(b);
	for( i=1;i<=n;i++);
	for(char j='a';j<='z';j++)
	{
		t=j;
		a1=a[i];
		b1=b[i];
		if(t==max(a1,b1))
		{
			c[i]=t;
			f=1;
		}
		continue;
		if(t=='z'&&f==0)
		{
			printf("-1");
			return 0;
		}
		
	}
	puts(c);
	return 0;
}
char max(char a,char b)
{
	if(a>b)
	return a;
	else 
	return b;
}