#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template<class T> bool cmin(T &i, T j) { return i > j ? i=j,true:false; }
template<class T> bool cmax(T &i, T j) { return i < j ? i=j,true:false; }
mt19937 mrand(chrono::steady_clock::now().time_since_epoch().count());
uniform_int_distribution<int> ui(0, 1 << 30);
int rng() {
return ui(mrand);
}
const int N=100100;
int n;
int h[N],k[N];
int placed[N];
int order[N];
bool done[N];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n;
for (int i=0;i<n;i++)
cin >> h[i] >> k[i];
iota(order,order+n,0);
sort(order,order+n,[&](int i,int j) {
return h[i] < h[j];
});
for (int t=0;t<n;t++) {
int i = order[t];
for (int j=0;j<h[i];j++)
done[j]=0;
while(k[i]--) {
int minp = -1;
for (int j=0;j<h[i];j++)
if (!done[j] && (minp==-1 || placed[j] < placed[minp]))
minp = j;
done[minp] = 1;
placed[minp]++;
}
}
ll ans = 0;
for (int i=0;i<N;i++) {
ans += 1LL * placed[i] * (placed[i] - 1) / 2;
}
cout << ans << endl;
}
