#include <bitset>
#include <cmath>
#include <functional>
#include <algorithm>
#include <numeric>
#include <deque>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <unordered_map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#include <cstring>
#include <climits>
#define int long long
#define pb push_back
#define MOD 1000000007
#define NMAX 250001
#define nl '\n'
#define INF 1000000007
#define pii1 pair<int, pair<int,int>> (1,(1,2));
#define pii pair<int,int>
#define tpl tuple<int,int,int>
using namespace std;
ifstream fin("data.in");
ofstream fout("data.out");
/*
--------------------DEMONSTRATION---------------------
3-2
5-3
4-1
2-1
4-3
3-2
subtask 1:1<=n<=15;
5*(5+1)/2=10;
1+2+3+4+5=15
------------------------END--------------------------
*/
class segtree{
private:
int n;
vector<int>tree;
public:
void init(int sz)
{
n=sz;
tree.resize(4*sz+1);
}
void update(int node,int st,int dr,int pos,int val)
{
if(st==dr)
{
tree[node]+=val;
return;
}
int mid=(st+dr)/2;
if(pos<=mid)
update(2*node,st,mid,pos,val);
else
update(2*node+1,mid+1,dr,pos,val);
tree[node]=tree[2*node]+tree[2*node+1];
}
pii combine(pii a,pii b)
{
if(a.first<b.first)
return a;
if(a.first==b.first)
{
if (a.second < b.second)
return a;
else
return b;
}
else
return b;
}
pii query(int node,int st,int dr,int L,int R)
{
if(st>R || dr<L)
return {INF,-1};
if(st==dr)
{
return {tree[node],st};
}
int mid=(st+dr)/2;
pii a=query(2*node,st,mid,L,R);
pii b=query(2*node+1,mid+1,dr,L,R);
return min(a,b);
}
void update(int pos,int val)
{
update(1,1,n,pos,val);
}
pii query(int L,int R)
{
return query(1,1,n,L,R);
}
};
int n,h,k;
int max_h;
vector<pii>v;
void subtask1()
{
map<int,int>mp;
for(int i=1;i<=max_h;++i)
{
mp[i]=0;
}
for(int i=1;i<=n;++i)
{
int maxi=v[i].first;
int cnt=v[i].second;
vector<int>aux;
for(auto [x,y]:mp)
{
if(x>maxi)
continue;
aux.pb(x);
}
auto cmp=[&](int a,int b)
{
if(mp[a]==mp[b])
return a>b;
return mp[a]<mp[b];
};
sort(aux.begin(),aux.end(),cmp);
for(auto x:aux)
{
if(!cnt)
break;
cnt--;
mp[x]++;
}
}
int ans=0;
for(int i=1;i<=max_h;++i)
{
// cout<<mp[i]-1<<" ";
if(mp[i])
{
mp[i]--;
ans+=(mp[i]*(mp[i]+1))/2;
}
}
cout<<ans;
}
segtree st;
void subtask2()
{
st.init(max_h);
int cnt=v[n].second;
for(int i=v[n].first;i>=1 && cnt;--i)
{
st.update(i,1);
cnt--;
}
// for(int i=1;i<=max_h;++i)
// {
// cout<<st.query(i,i).first<<nl;
// }
for(int i=n-1;i>=1;--i)
{
// cout<<i<<": ";
vector<pii>interval;
interval.pb({1,v[i].first});
for(int j=1;j<=v[i].second;++j)
{
// cout<<nl;
pii mini={INF,INF};
for(auto [x,y]:interval)
{
// cout<<x<<" "<<y<<" ";
mini=min(mini,st.query(x,y));
}
st.update(mini.second,1);
// cout<<mini.second<<" ";
for(int i=0;i<interval.size();++i)
{
auto [x,y]=interval[i];
if(mini.second>=x && mini.second<=y)
{
if(mini.second-1>=1 && x<=mini.second-1)
interval[i]={x,mini.second-1};
else
interval[i]={0,0};
if(mini.second+1<=max_h && mini.second+1<=y)
interval.pb({mini.second+1,y});
break;
}
}
}
// cout<<nl;
}
int ans=0;
for(int i=1;i<=max_h;++i)
{
int aux=st.query(i,i).first;
aux--;
// cout<<aux<<" ";
ans+=((aux*(aux+1))/2);
}
cout<<ans;
}
signed main() {
cin>>n;
v.resize(n+1);
for(int i=1;i<=n;++i)
{
cin>>h>>k;
v[i]={h,k};
max_h=max(max_h,h);
}
if(n<=15)
{
subtask1();
return 0;
}
else
{
subtask2();
}
return 0;
}
