def fun(n,a,b):
    p=[]
    p.append(a)
    p.append(a)
    l=[]
    l.append(a)
    l.append(b)
    for i in range(2,n-2):
        l.append(l[i-2]+l[i-1])
    for i in range(2,n):
        p.append(p[i-1]+l[i-2])
    return p[n-1]

if __name__ == '__main__':
    a,n,m,x=map(int,input().split())
    for i in range(1,100):
        if fun(n-1,a,i)==m:
            print(fun(x,a,i))
            break