string sub (string str, int start, int len) {
  var arr = array_of_string(str);
  var r = new int[len];
  for (var i = 0; i < len; i = i + 1;) {
      r[i] = arr[i+start];
  }
  return string_of_array (r);
}

int program (int argc, string []  argv) {
  print_string (sub(argv[1], 3, 5));
  return 0;
}