哪位大佬帮忙看一下

#include <bits/stdc++.h>
using namespace std;

int a,n,m,x;

int main(){
	cin>>a>>n>>m>>x;
	int upl=a;
	int upn=(m-a)/n;
	int down=(m-a)/n;
	int ans=upl;
	for(int i=3;i<=x;i++)
	{
		ans=ans+upl+upn-down;
		down=upl+upn;
		int temp=upl;
		upl=upn;
		upn+=temp; 
	}
	cout<<ans;
	return 0;
}