#include<stdio.h>
#include<string.h>
#include<ctype.h>
int main(){
    char s[101];
    int count[256]={0};
    fgets(s,sizeof(s),stdin);
    s[strcspn(s,"\n")]='\0';
    for(int i=0;s[i]!='\0';i++){
        if(!(isalnum(s[i]) || s[i] == ' ')){
            printf("Invalid input");
            return 0;
        }
        if(s[i]!=''){
            count[(unsigned char)s[i]]++;
        }
    }
    for(int i =0;s[i]!='\0';i++){
        if(s[i]!=' ' && count(unsigned char)s[1]==1){
            printf("%c",s[i]);
        }
    }
    return 0;
}