https://twitter.com/reiya_2200/status/1130761526959267841

sum.c

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


int main(int argc, char *argv[]) {
  if (0 < argc) {
    printf("%d\n", main(-argc, argv));
    return 0;
  }
  if (-argc - 1) {
    return atoi(argv[-argc - 1]) + main(argc + 1, argv);
  } else {
    return 0;
  }
}