I wrote simple implementation but can't figure out test to prove my solution wrong.

4 3 -> 4
3 3 -> 2
4 5 -> 6
4 6 -> 7
3 8 -> 8
6 5 -> 8
6 8 -> 10

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

int main(void) {
  int bigger, lesser, a, b;
  scanf("%d", &a);
  scanf("%d", &b);
  if (a >= b) {
    bigger = a - 1; lesser = b - 1;
  } else {
    bigger = b - 1; lesser = a - 1;
  }

  if (bigger % lesser == 0) {
    printf("%d\n", bigger); return 0;
  }

  printf("%d\n", bigger + lesser - 1);

  return 0;
}

Found it
7 11  -> 14
13 21 -> 16
GCD is the key ;-)

13 21 -> 28