hello all

given n cities from 1 to n and m flights.each flight is represented as(L,R,c) which means flight starts from city L and end at R and the cost is c(one can travel between any two cities from lower index to larger ,both inclusive)with cost c mathematically (l,r,c)--people can travel from u to v(l<=u<v<=r) at the cost c//not like our trains:)

find the minimum cost from city 1 to n,if not reachable o/p -1;

constraints: n<=1e5 m<=1e5 1<=l<r<=n c<=1e9;

i tried to create an adjacency list but to update the list i did not get idea how to do that effectively? bec it took (r-l)^2 for updating for each flight ,but that would tle