#include <stdio.h>                                                                                                                                         
#include <string.h>                                                                                                                                        
                                                                                                                                                           
int main(void)                                                                                                                                             
{                                                                                                                                                          
        char str[1000000 + 1] = { 0 };                                                                                                                     
        char *check = "abcdefghijklmnopqrstuvwxyz";                                                                                                        
        int len, i, j;                                                                                                                                     
                                                                                                                                                           
        scanf("%s", str);                                                                                                                                  
        len = strlen(str);                                                                                                                                 
                                                                                                                                                           
        for (i = 0; i < 26; i++) {                                                                                                                         
                for (j = 0; j < len; j++) {                                                                                                                
                        if (check[i] == str[j]) {                                                                                                          
                                break;                                                                                                                     
                        }                                                                                                                                  
                }                                                                                                                                          
                if (j == len) {                                                                                                                            
                        printf("%c\n", check[i]);                                                                                                          
                        return 0;                                                                                                                          
                }                                                                                                                                          
        }                                                                                                                                                  
        printf("None\n");                                                                                                                                  
        return 0;                                                                                                                                          
}

Measured memory_bytes: 1152