#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>

#define EPARSE ERANGE + 1

int sumacifp(int n) {
int t = 0;
int sum = 0;
while (n)
    {
    ++t;
if (t % 2 == 0)
{
sum += n % 10;
}
n /= 10;
    }
if (t < 2)
{
return -1;
}
return sum;
}

long int parse_long_int (const char* s) {
    char* str_end;
    long int n = strtol(s, &str_end, 10);
    /* if after the conversion str_end is not at the
     * end of the string, then the string is not a number
     */
    if (*str_end != '\0') {
        errno = EPARSE;
        return 0;
    }
    if (errno == ERANGE) {
        return 0;
    }

    return n;
}

void validate_arguments (int nargs, char* args[]) {
    const char how_to_use[] = "Usage: %s <num1>\nWhere <num1> is non-negative integer\n";
    const char too_many_args[] = "Too many arguments";
    const char too_few_args[] = "Not enough arguments";
    const char invalid_first[] = "Invalid argument";

    if (nargs < 2) {
        fprintf(stderr, "%s\n", too_few_args);
        fprintf(stderr, how_to_use, args[0]);
        exit(EXIT_FAILURE);
    }

    if (nargs > 2) {
        fprintf(stderr, "%s\n", too_many_args);
        fprintf(stderr, how_to_use, args[0]);
        exit(EXIT_FAILURE);
    }

    const char* arg = args[1];


    long int number = parse_long_int(arg);
    if (errno == ERANGE || errno == EPARSE || number < 0) {
        fprintf(stderr, "%s\n", invalid_first);
        fprintf(stderr, how_to_use, args[0]);
        exit(EXIT_FAILURE);
    }
}


int main (int nargs, char* args[]) {
    validate_arguments(nargs, args);

    long int number = parse_long_int(args[1]);

    long int sum = sumacifp(number);
    printf("%ld\n", sum);

    return 0;
}