sum from cl args

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

int main(int argc, char *argv[]) {
  if (argc & 1) {
    fprintf(stderr, "arg error!\n");
    exit(-1);
  }

  int sum = strtol(argv[1], NULL, 10);
  for (int i = 2; i < argc; i++) {
    int sign = 1, nr;
    if (*argv[i] == '-') {
      sign = -1;
    }
    i++;
    nr = strtol(argv[i], NULL, 10);
    nr *= sign;
    sum += nr;
  }
  printf("result: %d\n", sum);
  return 0;
}