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

int main() {
	int arr[]= {1, 5, 3, 4, 2};
	int n=sizeof(arr)/sizeof(arr[0]);
	int diff=2;
	int count=0;
	unordered_map<int, int> st;
	for(int j=0;j<n;j++){
		if(st.find(diff+arr[j])!=st.end())
			count=count+st[diff+arr[j]];
		st[arr[j]]++;
	}
	cout<<count;
}