70玄关求迢

#include<bits/stdc++.h>
using namespace std;
int main(){
	int a,b,m,n;
	unsigned long long s[1000],ans=0;
	cin>>a>>b>>m>>n;
	s[1]=a;
	s[2]=b;
	for(int i=3;i<=n;i++){
		s[i]=s[i-1]+s[i-2];
	}
	for(int i=1;i<=n;i++){
		if(s[i]>=m){
			cout<<ans+s[i];
			break;
		}
		ans+=s[i];
	}
	return 0;
}