#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define pb push_back
const int NUM_ROWS = 1e5;
struct Row {
int num_sails;
int row_id;
};
struct RowCustomComparator {
bool operator()(Row a, Row b) const {
if (a.num_sails == b.num_sails) {
return a.row_id < b.row_id;
}
return a.num_sails < b.num_sails;
}
};
vector<int> h, k;
set<Row, RowCustomComparator> rows;
int get_inef () {
int ans = 0;
for (auto e : rows) {
cout << "{" << e.row_id << ", " << e.num_sails << "}\n";
ans += e.num_sails;
}
return ans;
}
int main () {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n, i, j, max_height, ans;
Row row, min_row;
cin >> n;
h = vector<int>(n + 1);
k = vector<int>(n + 1);
max_height = 0;
for (i = 1; i <= n; i++) {
cin >> h[i] >> k[i];
max_height = max(max_height, h[i]);
}
for (i = 1; i <= max_height; i++) {
row = {0, i};
rows.insert(row);
}
ans = 0;
for (i = n; i >= 1; i--) {
// cout << get_inef() << "\n";
vector<bool> was_visited(h[i] + 1, false);
for (j = 1; j <= k[i]; j++) {
auto it = rows.begin();
while ((*it).row_id > h[i] || was_visited[(*it).row_id]) {
it++;
}
min_row = *it;
was_visited[min_row.row_id] = true;
ans += min_row.num_sails;
rows.erase(min_row);
min_row.num_sails++;
rows.insert(min_row);
}
// cout << "!! " << ans << " !!\n";
}
cout << ans << "\n";
return 0;
}
