#include<stdio.h>
#include<string.h>
int main(){
    char a[20];
    scanf("%s",a);
    int i,j,len=strlen(a);
    int found=0;
    for(int i=0;i<len;i++){
        int count=0;
        for(int j=0;j<len;j++){
            if(s[i]==s[j]){
                count++;
            }
            if(count==1){
                print("%c",s[i]);
                found = 1;
                break;
            }
        }
        if(!found)
        printf("-1");
        return 0;
    }
}