Domino on Torus

Imagine a stretchable square paper. Cut a rectangle of size $A \times B$ along the grid lines. All squares are numbered from 1 to $A \cdot B$. From this rectangle, cut another rectangle of size $C \times D$ along the grid lines, so that the sides of length $C$ are parallel to the sides of length $A$. Then glue the sides of length $B$ together, and finally glue the sides of length $A$ together. The result is a torus with a rectangular hole of $C \times D$ squares. (A torus is the surface of a bagel.)

The squares of the torus are considered different if they have different numbers.

We now tile the external surface of the torus with the hole using stretchable dominoes. Each domino consists of two squares of two different colors, one white and one black, that share a common side.

The tiling must satisfy this condition: if two squares of the torus that are adjacent by side belong to different dominoes, they must be the same color, both white or both black.

Two tilings are different if at least one of the following holds:

1. at least one square of the torus is white in one tiling and black in the other;
2. at least one square is covered by a domino whose other square is $Y_1$ in one tiling and $Y_2$ in the other, with $Y_1 \ne Y_2$.

The first line of input contains four integers $A$, $B$, $C$ and $D$ ($4 \le A, B \le 10^9$, $2 \le C < A$, $2 \le D < B$, all numbers are even).

Output the number of tilings.

The following picture shows one of the possible tilings in the given example.