#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
//#pragma GCC target("avx,avx2,fma")
#define int long long 
#define F first
#define S second 
#define pb push_back
#define endl '\n'
#define all(v) v.begin(),v.end()
#define gcd(a,b) __gcd(a,b)
#define mt make_tuple
#define pqueue priority_queue
typedef pair<int,int> ii;
typedef tuple<int,int,int> iii;
typedef tuple<int,int,int,int> iiii;
typedef vector<int> vi;
typedef vector<bool> vb;
typedef vector<string> vs;
typedef vector<char> vc;
typedef vector<iii> viii;
typedef set<int> si;
typedef vector<ii> vii;				
typedef vector<vi> vvi;
typedef vector<si> vsi;
typedef vector<vb> vvb;
typedef vector<vc> vvc;
const int MOD = 998244353; 
const int N = 1e6 + 100; 
int n; 
vii mast; 
int32_t main(){
	ios_base::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);
	
	cin >> n;
	mast.assign(n,{});  
	for (int i = 0; i < n; i++) cin >> mast[i].F >> mast[i].S; 
	sort(all(mast));
	pqueue<int,vi,greater<int>> pq; 
	int ans = 0; 
	for (int i = 0; i < n; i++){
		while (pq.size() < mast[i].F) pq.push(0);
		vi tmp;  
		for (int j = 0; j < mast[i].S; j++){
			tmp.pb(pq.top()); 
			pq.pop(); 
		}
		for (int j = 0; j < mast[i].S; j++){
			pq.push(tmp[j] + 1); 
		} 
	}
	while (!pq.empty()) {
		ans += pq.top() * (pq.top() - 1) / 2; 
		pq.pop(); 		
	}
	cout << ans << endl; 
	return 0;
}	
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