Travelling Saleschef

Chef works in a similar way to a travelling salesman ― he always travels to new cities in order to sell his delicious dishes. Today, Chef is planning to visit $N$ cities (numbered $1$ through $N$). There is a direct way to travel between each pair of cities. Each city has a specific temperature; let's denote the temperature in the $i$-th city by $C_i$. Chef has a fixed temperature tolerance $D$ w