diff --git a/Muons/test/ex3_1.py b/Muons/test/ex3_1.py
index 3549544..f34e7e2 100644
--- a/Muons/test/ex3_1.py
+++ b/Muons/test/ex3_1.py
@@ -92,7 +92,7 @@
     phIso = probe.photonIso()
     puIso = probe.puChargedHadronIso()
 
-    relIso = (chIso+nhIso+phIso+0.5*max(0, -puIso))/probe.pt()
+    relIso = (chIso+ max(0, nhIso+phIso-0.5*puIso))/probe.pt()
 
     ntuple.Fill(z.mass(), tag.pt(), tag.eta(),
                 probe.pt(), probe.eta(),