#include <bits/stdc++.h>
#define pb push_back
#define rs resize
#define debug printf("Hello\n")
#define Pi 3.141592653589793
#define sz(a) ll((a).size())
#define all(x) (x).begin(), (x).end()
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define endl "\n"
#define mp make_pair
#define f first
#define s second
#define vt vector
#define rst(a,b) memset((a),(b), sizeof(a))
#define FOR(a, b, c) for (ll a = (b); (a) < (c); ++(a))
#define FORE(a, b, c) for (ll a = (b); (a) <= (c); ++(a))
#define FORR(a, b, c) for (ll a = (b); (a) >= (c); --(a))
#define umap unordered_map
#define len(a) (a).length()
#define pqueue priority_queue
using namespace std;
using vi = vector<int>;
using ui = unsigned int;
using ll = long long;
using pll = pair<ll,ll>;
using vll = vector<ll>;
using ull = unsigned long long;
using pii = pair<int, int>;
const int dx[4] = {0,0,-1,1};
const int dy[4] = {1,-1,0,0};
const char dir[4] = {'R', 'L', 'U', 'D'};
const int mod = 1e9+7;
const int inf = 0x3f3f3f3f;
const ll INF = 0x3f3f3f3f3f3f3f3f;
void solve(){
ll n;cin >> n;
vt<pll> arr(n);
FOR(i,0,n){
ll a,b;cin >> a >> b;
arr[i] = mp(a,b);
}
sort(all(arr));
multiset<pll> q;
ll t = arr[0].s;
FOR(i,0,arr[0].f){
if(t-->0)q.insert(mp(1,i));
else q.insert(mp(0,i));
}
FOR(i,1,n){
ll tmp = arr[i].s;
queue<pll> toadd;
if(q.size() < arr[i].f){
FOR(j,q.size(),arr[i].f){
q.insert(mp(0,j));
}
}
while(tmp--){
pll ff = *q.begin();
q.erase(q.begin());
toadd.push(mp(ff.f+1, ff.s));
}
while(toadd.size()){
q.insert(toadd.front());
toadd.pop();
}
}
ll sum = 0;
for(auto x : q){
ll ff = x.f;
sum += (ff)*(ff-1)/2;
}
cout << sum << endl;
}
int main(){
fastio;
solve();
}
