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

int main() {
    int soucet = 0, licho;
    int c;
    
    int pole[64], * p; // kdyz bude ve vstupu vic jak 64 cislic, muze to spadnout
    
    while (1) {
        p = pole, soucet = 0;
        
        printf("Zadej cislo vozidla (nebo nic -> konec): ");
    
        while ((c = getchar()) != '\n') {
            if (isdigit(c)) {
                *p++ = c - '0';
            }
        }
        if (p == pole) return 0;
        p--;
         
        for (licho = 1; p >= pole; p--, licho++) {
            if (licho % 2 == 1) *p *= 2;
            if (*p > 9) *p -= 9;
            soucet += *p;
        }
        printf("kontrolni cislice: %d\n", ((10 - (soucet % 10)) % 10));
    }
    
}