#include <bits/stdc++.h>
#define F first
#define S second
#define PB push_back
#define debug(x) cout<<#x<<" = "<<(x)<<"\n";
using namespace std;
void print(vector<vector<int>>& boat) {
cout<<"elements in boat\n";
for (int i = 1; i <= 5; ++i) {
for (int j = 1; j <= 6; ++j) {
if (boat[i][j]==-1) cout<<7<<' ';
else cout<<boat[i][j]<<' ';
} cout<<"line "<<5 - i + 1<<'\n';
}
}
void print2(vector<pair<int, int>>& v) {
cout<<"elements in pairs v\n";
for (auto &[a,b] : v)
{
cout<<a<<' '<<b<<' '<<" | ";
} cout<<"\n";
}
bool customsort(pair<int, int>& p1, pair<int, int>& p2) {
if (p1.F != p2.F) return p1.F < p2.F;
return p1.S > p2.S;
}
int main() {
ios_base::sync_with_stdio(0);cin.tie(0);
int n; cin>>n;
vector<pair<int, int>> v(n + 1);
int mx = 1;
for (int i = 1; i <= n; ++i) {
int h, k; cin>>h>>k;
v[i] = {h, k};
mx = max(mx, h);
}
vector<vector<int>> boat(mx + 1, vector<int>(n + 2, -1));
int tnf = 0;
for (int p = n; p >= 1; --p) {
int pl = n - p + 1;
//debug(pl);
vector<pair<int, int>> s;
for (int h = v[p].F; h >= 1; --h) {
int nh = mx - h + 1;
s.PB({boat[nh][pl - 1], h});
//debug(h)
//debug(boat[nh][pl - 1])
}
sort(s.begin(), s.end(), customsort);
//print2(s);
for (auto &[val, h] : s) {
int nh = mx - h + 1;
if (v[p].S == 0) break;
boat[nh][pl] = val + 1;
for (int pll = pl + 1; pll <= n; ++pll) boat[nh][pll] = val + 1;
v[p].S--;
tnf += boat[nh][pl];
}
//print(boat);
}
cout<<tnf<<'\n';
}
