P1011 [NOIP1998 提高组] 车站

#include <bits/stdc++.h>
#define int long long
using namespace std;
int a[21];
int fbnq(int x){
	if (x <= 2)
		return 2;
	else if (a[x])
		return a[x];
	else {
		int p = fbnq(x - 1) + fbnq(x - 2);
		a[x] = p;
		return p;
	}
}
signed main(){
	int a, n, m, x;
	cin >> a >> n >> m >> x;
	int k = (m - a) / fbnq(n - 3);
	if (x <= 3)
		cout << a;
	else
		cout << a + fbnq(x - 3) * k;
	return 0;
}