They're dealing with planar graphs, graphs embedded in planes. Faces are not edges. https://en.wikipedia.org/wiki/Planar_graph#Euler's_formula https://en.wikip

They're dealing with planar graphs, graphs embedded in planes. Faces are not edges. https://en.wikipedia.org/wiki/Planar_graph#Euler's_formula https://en.wikipedia.org/wiki/Planar_graph#Euler's_formula