#include<iostream>
using namespace std;
const int N = 2e4; 
int main()
{
    int s1[N] = {0},s2[N] = {0};
    int a,n,m,x;
    cin >> a >> n >> m >> x;
    s1[1] = 1;
	s1[2] = 1;
	s1[3] = 2;
    for(int i = 4;i < n;i++)
        s1[i] = s1[i - 1] + s1[i - 2] -1,s2[i] = s2[i - 1] + s2[i - 2] + 1;
    int b = (m - a * s1[n - 1]) / s2[n - 1];
    cout << s1[x] * a + s2[x] * b << endl;
    return 0;
}

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