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B. Covered Path

time limit per test

1 secondmemory limit per test

256 megabytesinput

standard inputoutput

standard outputThe on-board computer on Polycarp's car measured that the car speed at the beginning of some section of the path equals *v*_{1} meters per second, and in the end it is *v*_{2} meters per second. We know that this section of the route took exactly *t* seconds to pass.

Assuming that at each of the seconds the speed is constant, and between seconds the speed can change at most by *d* meters per second in absolute value (i.e., the difference in the speed of any two adjacent seconds does not exceed *d* in absolute value), find the maximum possible length of the path section in meters.

Input

The first line contains two integers *v*_{1} and *v*_{2} (1 ≤ *v*_{1}, *v*_{2} ≤ 100) — the speeds in meters per second at the beginning of the segment and at the end of the segment, respectively.

The second line contains two integers *t* (2 ≤ *t* ≤ 100) — the time when the car moves along the segment in seconds, *d* (0 ≤ *d* ≤ 10) — the maximum value of the speed change between adjacent seconds.

It is guaranteed that there is a way to complete the segment so that:

- the speed in the first second equals
*v*_{1}, - the speed in the last second equals
*v*_{2}, - the absolute value of difference of speeds between any two adjacent seconds doesn't exceed
*d*.

Output

Print the maximum possible length of the path segment in meters.

Examples

Input

5 6

4 2

Output

26

Input

10 10

10 0

Output

100

Note

In the first sample the sequence of speeds of Polycarpus' car can look as follows: 5, 7, 8, 6. Thus, the total path is 5 + 7 + 8 + 6 = 26 meters.

In the second sample, as *d* = 0, the car covers the whole segment at constant speed *v* = 10. In *t* = 10 seconds it covers the distance of 100 meters.

Codeforces (c) Copyright 2010-2017 Mike Mirzayanov

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