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

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