#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int main() {
ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
int n; cin >> n;
vector<pair<int, int>> v(n);
for (int i = 0; i < n; i ++) cin >> v[i].first >> v[i].second;
sort(v.begin(), v.end());
vector<int> db(v[n - 1].first);
for (int i = 0; i < n; i ++) {
int ind = db.size() - (v[i].first - v[i].second) - 1;
int x = db[ind];
vector<int> db2;
db2.assign(db.begin(), db.begin() + v[i].first);
reverse(db2.begin(), db2.end());
int a = upper_bound(db2.begin(), db2.end(), x) - db2.begin();
int b = lower_bound(db2.begin(), db2.begin(), x) - db2.begin();
a = v[i].first - a;
b = v[i].first - b - 1;
int cnt = 0;
for (int j = b + 1; j < v[i].first; j ++) {
db[j] ++; cnt ++;
}
for (int j = a; j < a + v[i].second - cnt; j ++) db[j] ++;
}
ll ans = 0;
for (int c : db) ans += c * (c - 1) / 2;
cout << ans;
return 0;
}
