#include<cstdio>
int main(){
	int y[21]={0,0,1,2,2,3,4,6,9,14,22,35,56,90,145,234,378,611,988,1598,2585};
	int z[21]={0,0,0,0,1,2,4,7,12,20,33,54,88,143,232,376,609,986,1596,2583};
    int a,n,m,x;
    scanf("%d%d%d%d",&a,&n,&m,&x);
    int X=(m-a*y[n-1])/z[n-1];
    printf("%d",a*y[x]+X*z[x]);
    return 0;
}