a

#include <cstring>
#include <cstdio>

char a[100];
char b[100];
char c[100];

int main() {
    scanf("%s %s", a,b);
    int len = strlen(a);
    int i;
    int x,y;
    strcpy(c,b);

    for(i=len-1;i>=0;i--) {
        if (c[i] != 'a') {
            c[i] -=1;
            break;
        }
    }
    x = strcmp(c,a);
    y = strcmp(c,b);

    if (x>0 && y<0) printf("%s\n",c);
    else {
        strcpy(c,a);
        for(i=len-1;i>=0;i--) {
            if (c[i] != 'z') {
                c[i] +=1;
                break;
            }
        }
        x = strcmp(c,a);
        y = strcmp(c,b);
        if (x>0 && y<0) printf("%s\n",c);
        else printf("No such string\n");
    }
    return 0;
}