diff --git a/manpages/shorewall-zones.xml b/manpages/shorewall-zones.xml
index 60e7f0f03..a1bbab69b 100644
--- a/manpages/shorewall-zones.xml
+++ b/manpages/shorewall-zones.xml
@@ -78,7 +78,7 @@
                 <simplelist>
                   <member>12 + 6 + 2*M + 1 = 29 which reduces to</member>
 
-                  <member>2*M = 29 - 12 - 6 = 11 or</member>
+                  <member>2*M = 29 - 12 - 6 - 1 = 10 or</member>
 
                   <member>M = 5</member>
                 </simplelist>
diff --git a/manpages6/shorewall6-zones.xml b/manpages6/shorewall6-zones.xml
index 79b0e4e5b..304c30c92 100644
--- a/manpages6/shorewall6-zones.xml
+++ b/manpages6/shorewall6-zones.xml
@@ -78,7 +78,7 @@
                 <simplelist>
                   <member>12 + 6 + 2*M + 1 = 29 which reduces to</member>
 
-                  <member>2*M = 29 - 12 - 6 = 11 or</member>
+                  <member>2*M = 29 - 12 - 6 - 1 = 10 or</member>
 
                   <member>M = 5</member>
                 </simplelist>