#include <stdio.h>
#include <strinh.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]]==1)
    }
    if(freq[(int)s[i]]);
    {
        printf("%c\n",s[i]);
        found = 1;
        break;
    }
}
if(!found)
{
    print("-1\n");
}
return 0;
}