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

char *interleave(char *str1, size_t len1, char *str2, size_t len2) {
  char *combined = malloc(len1 + len2 + 1);

  if(!combined) {
    return NULL;
  }

  char *idx = combined;

  while(*str1 && *str2) {
    *idx = *str1, *(idx + 1) = *str2;
    idx += 2, str1 += 1, str2 += 1;
  }

  char *rest = str1;

  if(*str2) {
    rest = str2;
  }

  strcat(idx, rest);
  return combined;
}

int main(int argc, char **argv) {
  if(argc != 3) {
    return 1;
  }

  size_t len1 = strlen(argv[1]);
  size_t len2 = strlen(argv[2]);

  char *combined = interleave(argv[1], len1, argv[2], len2);

  if(!combined) {
    return 1;
  }

  printf("%s\n", combined);
  free(combined);
  return 0;
}