# -*- coding: utf-8 -*-
bookWidth, borderWidth, firstId, lastId = [int(i) for i in input().split()]
if lastId > firstId:
    if lastId - firstId == 1:
        print(borderWidth * (lastId - firstId + 1) + bookWidth * (lastId - firstId - 1))
    else:
        print(borderWidth * (lastId - firstId + 2) + bookWidth * (lastId - firstId - 1))
elif lastId < firstId:
    if firstId - lastId == 1:
        print(bookWidth * (firstId - lastId + 1) + borderWidth * (firstId - lastId + 1))
    else:
        print(bookWidth * (firstId - lastId + 1) + borderWidth * (firstId - lastId + 2))
else:
    print(bookWidth)