10分疯了

#include<bits/stdc++.h>
#define ll long long
using namespace std;
ll n,m,a,b,cnt,ans;
int main() {
  cin>>a>>b>>m>>n;
  ans+=cnt+=a+b;
  for(int i=2;i<=n;i++) {
    int x=cnt;
		cnt+=b;
		b=x;
    if(cnt>=m) {
			cout<<ans;
			return 0;
		}
		ans+=cnt;
	}
	cout<<ans;
	return 0;
}