#include "bits/stdc++.h"
using namespace std;
#ifdef DEBUG
auto&operator<<(auto &o, pair<auto, auto> p) {o << "(" << p.first << ", " << p.second << ")"; return o;}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{"; for(auto e : x) o<<e<<", "; return o<<"}";}
#define debug(X) cerr << "["#X"]: " << X << '\n';
#else
#define cerr if(0)cout
#define debug(X) ;
#endif
using ll = long long;
#define all(v) (v).begin(), (v).end()
#define ssize(x) int(x.size())
#define fi first
#define se second
#define mp make_pair
#define eb emplace_back
const ll inf = ll(1e12);
int main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
int n;
cin >> n;
vector<int> a(n), b(n);
#ifndef DEBUG
const int H = 1'003;
#else
const int H = 10;
#endif
vector<int> x(H);
for (int i = 0; i < n; ++i) {
cin >> a[i] >> b[i];
for (int j = a[i] - 1; j >= a[i] - b[i]; --j) {
x[j]++;
}
}
debug(x);
vector<ll> pref(H);
pref[0] = x[0];
for (int i = 1; i < H; ++i) pref[i] = x[i] + pref[i-1];
for (int k = H-1; k >= 0; --k) {
ll c = inf;
for (int j = 0; j <= k; ++j) {
ll s = ll(x[k]) + (k-1 < 0 ? 0 : pref[k-1]) - (j-1 < 0 ? 0 : pref[j-1]);
c = min(c, s / (k - j + 1));
}
debug(k);
debug(c);
if (k) x[k-1] += x[k] - int(c);
x[k] = int(c);
}
debug(x);
ll res = 0;
for (int k = 0; k < H; ++k)
res += ll(x[k]) * (x[k] - 1) / 2;
cout << res << '\n';
return 0;
}
