Making Grid Paper

Someone owns many clean white sheets of paper, all of different sizes. They want to turn one of the sheets into grid (graph) paper by drawing some vertical lines and some horizontal lines parallel to the sheet's edges, splitting it into small rectangles.

The gap between any two neighboring lines must be at least $d$ millimeters, and every line must be at least $1$ millimeter away from each edge of the sheet. They want to pick the sheet that can be split into the largest number of rectangles (the rectangles formed between an outermost line and an edge of the sheet are counted too). If several sheets can be split into the same largest number of rectangles, they pick the one with the greatest area. Find the area of the chosen sheet.

The first line contains the number of white sheets $n$ and the minimum gap between lines $d$ ($1 \le n \le 10^6$, $1 \le d \le 100$). Each of the next $n$ lines describes one sheet with two integers $a_i$ and $b_i$, the width and height of the $i$-th sheet ($2 \le a_i, b_i \le 10^4$).

Print a single integer: the area of the sheet that should be chosen.