diff --git a/tests/cidr4_merge/test_cidr4_merger.py b/tests/cidr4_merge/test_cidr4_merger.py
index 319976b..32add5f 100644
--- a/tests/cidr4_merge/test_cidr4_merger.py
+++ b/tests/cidr4_merge/test_cidr4_merger.py
@@ -6,6 +6,7 @@ from vpn_manager.cidr4_merge.cidr4_merger import (
     cidr4_to_node,
     find_parent,
     make_cidr4,
+    merge_two_nodes,
     sort_nodes,
 )
 
@@ -88,13 +89,20 @@ def test_find_parent__with_exception():
 
 
 def test_calc_dip():
+    assert calc_dip(32, 32) == 0
+    assert calc_dip(32, 31) == 2
+    assert calc_dip(32, 30) == 4
+    assert calc_dip(31, 30) == 2
+    assert calc_dip(30, 29) == 4
+    assert calc_dip(29, 31) == 6
     assert calc_dip(26, 26) == 0
+    assert calc_dip(3, 2) == 2**30 - 2**29
+    assert calc_dip(3, 1) == 2**31 - 2**29
 
-    assert calc_dip(26, 27) == 67108864
-    assert calc_dip(27, 26) == 67108864
 
-    assert calc_dip(26, 28) == 201326592
-    assert calc_dip(28, 26) == 201326592
-
-    assert calc_dip(26, 29) == 469762048
-    assert calc_dip(29, 26) == 469762048
+def test_merge_two_nodes():
+    assert merge_two_nodes((0, 32), (1, 32)) == ((0, 31), 0)
+    assert merge_two_nodes((0, 32), (2, 32)) == ((0, 30), 4)
+    assert merge_two_nodes((0, 32), (5, 32)) == ((0, 29), 8)
+    assert merge_two_nodes((3, 32), (4, 32)) == ((0, 29), 8)
+    assert merge_two_nodes((0, 32), (4, 30)) == ((0, 29), 4)
diff --git a/vpn_manager/cidr4_merge/cidr4_merger.py b/vpn_manager/cidr4_merge/cidr4_merger.py
index a2d7802..c3bfeab 100644
--- a/vpn_manager/cidr4_merge/cidr4_merger.py
+++ b/vpn_manager/cidr4_merge/cidr4_merger.py
@@ -43,14 +43,25 @@ def find_parent(a: Node, b: Node) -> Node:
 
 
 def calc_dip(mask_len_a: int, mask_len_b: int) -> int:
-    dip = 0
-    len_min, len_max = sorted((mask_len_a, mask_len_b))
-    while len_min < len_max:
-        dip += 1 << len_min
-        len_min += 1
+    # a = 2 ** (32 - mask_len_a) if mask_len_a != 32 else 0
+    # b = 2 ** (32 - mask_len_b) if mask_len_b != 32 else 0
+    a = 1 << (32 - mask_len_a) if mask_len_a != 32 else 0
+    b = 1 << (32 - mask_len_b) if mask_len_b != 32 else 0
+    dip = abs(a - b)
     return dip
 
 
+def merge_two_nodes(node_a: Node, node_b: Node) -> tuple[Node, int]:
+    ip_a, mask_len_a = node_a
+    ip_b, mask_len_b = node_b
+    parent_node = find_parent(node_a, node_b)
+    _, mask_len_p = parent_node
+    dip_a = calc_dip(mask_len_a, mask_len_p + 1)
+    dip_b = calc_dip(mask_len_b, mask_len_p + 1)
+    dip = dip_a + dip_b
+    return parent_node, dip
+
+
 def make_cidr4(ip, mask_len) -> str:
     lst = [str(ip >> (i << 3) & 0xFF) for i in reversed(range(4))]
     ip_address = ".".join(lst)