#include <bits/stdc++.h>
using namespace std;
int main()
{
int a, b, n, m, c;
int ans;
cin >> a >> b >> m >> n;
ans = a + b;
for (int i = 3; i <= n; i++)
{
c = a + b;
ans += c;
a = b;
b = c;
if (c >= m)

{
break;
}
}
if (n == 1)
{
ans = a;
}
if (n == 2)
{
ans = a + b;
}
cout << ans;
return 0;
}