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