from sympy import *
a, n, m, x = map(int,input().split())
lst1 = []
c = Symbol('c')
for i in range(1,n+1):
    tem = []
    tem.append(i)
    if i == 1:
        tem.extend((a,a,0))
        lst1.append(tem)
    elif i == 2:
        tem.extend((a,c,c))
        lst1.append(tem)
    elif i == n:
        tem.extend((0,0,m))
        lst1.append(tem)
    else:
        sha = lst1[i-3][2] + lst1[i-2][2]
        xia = lst1[i-2][2]
        she = lst1[i-2][1] + sha - xia
        tem.extend((she,sha,xia))
        lst1.append(tem)

dic1 = solve([lst1[n - 2][1] - m], [c])
k = dic1[c]
qiu = lst1[x-1][1]
qiu1 = qiu.replace(c, k)
print(int(qiu1))