#include <iostream>
#include<vector>
using namespace std;

int main() {
	vector<int> nums={1,3,4,5,8};
	int st=0;
	int end=nums.size()-1;
	int count=0;
	int k=5;
	
	while(st<end) {
		if(nums[st]+nums[end]>k) {
			count+=end-st;
			end--;
		}
		else {
			st++;
		}
	}
	
	cout<<count<<endl;
	
	return 0;
}