Add total video length to complex

src/1_complex_numbers.md

@@ -4,6 +4,10 @@ title: Complex Numbers
 
 # Complex numbers
 
+The lecture on complex numbers consists of three parts, each with their own video and text.
+
+**Total video length: 38 minutes and 53 seconds**
+
 ## Definition and basic operations
 
 <iframe width="100%" height=315 src="https://www.youtube-nocookie.com/embed/fLMdaMuEp8s" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>