up_b2=0
up_b1=0
up=0
s=0
input_data= input()
a,n,m,x= input_data.split( )
a=int(a)
n=int(n)
m=int(m)
x=int(x)
s=a
if x<=2 :
    print(a)
else:
    for i in range(1,a+1,1) :
        up_b2=a
        up_b1=i
        s=a
        for j in range(3,n) :
            up=up_b1+up_b2
            s=s+up_b2
            up_b2=up_b1
            up_b1=up
        if s==m:
            up_b2=a
            up_b1=i
            break
    s=a
    for j in range(3,x+1) :
        up=up_b1+up_b2
        s=s+up_b2
        up_b2=up_b1
        up_b1=up
    print(s)