diff --git a/preliminaries/motifs-and-structral-roles_lecture.md b/preliminaries/motifs-and-structral-roles_lecture.md
index fd435de..df82f09 100755
--- a/preliminaries/motifs-and-structral-roles_lecture.md
+++ b/preliminaries/motifs-and-structral-roles_lecture.md
@@ -43,7 +43,7 @@ Configuration model is a random graph with a given degree sequence $$k_1$$, $$k_
 
 Another way for generation is as following:
 1) start from a given graph G;
-2) select a pair of edges A->B, C->D at random, exchange the endpoints to give A->D, C->B, repeat the switching step Q* $$\vert$$ E $$\vert$$ times.
+2) select a pair of edges A->B, C->D at random, exchange the endpoints to give A->D, C->B, repeat the switching step Q* $$\vert E\vert$$ times.
 
 By this way, we will get a randomly rewired graph with same node degrees and randomly rewired edges.