Two graphs $\Gamma_1$ and $\Gamma_2$ are called isomorphic if there is a bijection $f\colon V(\Gamma_1)\to V(\Gamma_2)$ such that vertices $A$ and $B$ of graph $\Gamma_1$ are connected with an edge if and only if vertices $f(A)$ and $f(B)$ are connected by an edge in $\Gamma_2$.