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

int main()
{
    char s[101];
    int freq[256] ={0};
    int i;
    int found = 0;
    scanf("%s",s);
    for(int i=0;i<strlen(s);i++)
    {
        freq[(int)s[i]]++;
    }
    for(int i=0;i<strlen(s);i++)
    {
        if(freq[(int)s[i]]==1)
        {
            printf("%c\n",s[i]);
            found = 1;
            break;
        }
    }
    if(!found)
    {
        print("-1\n");
    }
return 0;
}