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

typedef int32_t i32;
typedef int64_t i64;

#define MAX(a,b) ((a) > (b) ? (a) : (b))
#define MIN(a,b) ((a) < (b) ? (a) : (b))

void run (void) {
  i32 a, b;
  scanf ("%" SCNi32 "%" SCNi32, &a, &b);
  i32 x = MIN(a, b);
  i32 y = MAX(a, b);
  i32 c = y - x;
  if ((x + y) % c == 0 && (c + y) % x == 0) {
    printf ("%" PRIi32 "\n", c);
    return;
  }
  c = x + y;
  if (((i64) c + x) % y == 0 && ((i64) c + y) % x == 0) {
    printf ("%" PRIi32 "\n", c);
    return;
  }
  puts("-1");
}

int main (void) {
  run();
  return 0;
}