## USACO 2020 December Contest, Platinum

## Problem 3. Cowmistry

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NOTE: Here, $a\oplus b$ denotes the "bitwise exclusive or'' of non-negative integers $a$ and $b$. This operation is equivalent to adding each corresponding pair of bits in base 2 and discarding the carry. For example,

Bessie has $N$ ($1\le N\le 2\cdot 10^4$) boxes of cow-michals and the $i$-th box contains cow-michals labeled $l_i$ through $r_i$ inclusive $(0\le l_i \le r_i \le 10^9)$. No two boxes have any cow-michals in common. She wants to know how many unique mixtures of three different cow-michals she can create. Two mixtures are considered different if there is at least one cow-michal present in one but not the other. Since the answer may be very large, report it modulo $10^9 + 7$.

#### INPUT FORMAT (input arrives from the terminal / stdin):

The first line contains two integers $N$ and $K$.Each of the next $N$ lines contains two space-separated integers $l_i$ and $r_i$. It is guaranteed that the boxes of cow-michals are provided in increasing order of their contents; namely, $r_i<l_{i+1}$ for each $1\le i<N$.

#### OUTPUT FORMAT (print output to the terminal / stdout):

The number of mixtures of three different cow-michals Bessie can create, modulo $10^9 + 7$.#### SAMPLE INPUT:

1 13 0 199

#### SAMPLE OUTPUT:

4280

We can split the chemicals into 13 groups that cannot cross-mix: $(0\ldots 15)$, $(16\ldots 31)$, $\ldots$ $(192\ldots 199)$. Each of the first twelve groups contributes $352$ unique mixtures and the last contributes $56$ (since all $\binom{8}{3}$ combinations of three different cow-michals from $(192\ldots 199)$ are okay), for a total of $352\cdot 12+56=4280$.

#### SAMPLE INPUT:

6 147 1 35 48 103 125 127 154 190 195 235 240 250

#### SAMPLE OUTPUT:

267188

#### SCORING

- Test cases 3-4 satisfy $\max(K,r_N)\le 10^4$.
- Test cases 5-6 satisfy $K=2^k-1$ for some integer $k\ge 1$.
- Test cases 7-11 satisfy $\max(K,r_N)\le 10^6$.
- Test cases 12-16 satisfy $N\le 20$.
- Test cases 17-21 satisfy no additional constraints.

Problem credits: Benjamin Qi