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

int main()
{
    char s[101], output;
    int seen = {0}; // ASCII table size
    int i, j = 0;
    int valid = 1;

    // Read input (allows spaces)
    fgets(s, 101, stdin);
    // Remove trailing newline if present
    size_t len = strlen(s);
    if (len > 0 && s[len-1] == '\n') s[len-1] = '\0';

    // Check for invalid characters
    for (i = 0; s[i]; i++) {
        if (!(isalpha(s[i])  isdigit(s[i])  s[i] == ' ')) {
            valid = 0;
            break;
        }
    }

    if (!valid) {
        printf("Invalid input\n");
        return 0;
    }

    // Remove duplicates, preserve order
    for (i = 0; s[i]; i++) {
        if (!seen[(unsigned char)s[i]]) {
            seen[(unsigned char)s[i]] = 1;
            output[j++] = s[i];
        }
    }
    output[j] = '\0';

    printf("%s\n", output);

    return 0;
}