Silent People

Chefland consists of $N$ cities. Each city has its own population. The population of the $i$th city is denoted by $P_i$. These people are weird as they do not talk to people who belong to the same city that they are in (That means for every $i$, people of the city $i$ do not talk with each other). People from a city talk with only people from their friend cities. Say city $i$ and city $j$ are fri