#include<stdio.h>
#include<string.h>
int main(){
    char s[101];
    int freq[256]={0};
    int i,len,found=0;
    scanf("%s",s);
    len = strlen(s);
    for(i =0;i<len;i++){
        freq[(unsigned char)s[i]]++;
        for(i=0;i<len;i++){
            if(freq[(unsigned char)s[i]]==1){
                printf("%c",s[i]);
                found=1;
                break;
            }
   }
 }
    if(!found)
    printf("-1");
    return 0;
}