Statement

After many years of obscurity, the well-known mobile game Flappy Bird has finally become downloadable again and Stefan decided to install it on his mobile phone in a quest to relive his old childhood days. Unlike the original version of the game, in this new edition the towers are oriented horizontally, rather than vertically. The bird needs to fly through N pairs of towers, represented as a grid of N rows and a (potentially very large) number of columns. Each of the N rows in the grid is described as an interval: two positions A[i] and B[i] which indicate that the bird can fly through those columns (both extremes are included). There is a problem though: given that the rows have no space between them (unlike the image above!), some levels of the game are impossible to solve! For simplicity, let's assume that the bird starts before the first row, can fly horizontally instantaneously...