#include <bits/stdc++.h>
using namespace std;
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//#pragma GCC optimize("Ofast,unroll-loops,fast-math,O3")
#define pb push_back
#define ll long long
#define f first
#define s second
//#define sz(v) int(v.size())
#define all(v) v.begin(),v.end()
int mod = 1e9 + 7;
const int N = 2e5,k = 317 + 1;
const int inf = 2e9;
void solve(){
int n;
cin >> n;
vector <pair<int,int>> v;
if(n <= 20){
int x,y;
for(int i = 1; i <= n; i++){
cin >> x >> y;
v.pb({x,y});
}
int ans = inf;
for(int mask = 0; mask < (1 << n); mask++){
vector <int> v1;
for(int i = 0; i < n; i++){
if((mask >> i & 1) == 1){
v1.pb(i);
}
}
bool f1 = 1;
for(int i = 0; i < n; i++){
int x1,y1;
bool f = 0;
for(auto to : v1){
x1 = v[to].f,y1 = v[to].s;
if(abs(x1 - v[i].f) <= y1 - v[i].s){
f = 1;
break;
}
}
if(f == 0){
f1 = 0;
}
}
if(f1 == 1){
ans = min(ans,__builtin_popcount(mask));
}
}
cout << ans;
return;
}
int cnt = 0,x,y,ans = 0,last = -1;
bool f = 0;
for(int i = 1; i <= n; i++){
cin >> x >> y;
if(y == 1) cnt++;
ans++;
if(y == 1 && last == 0 && f == 0){
f = 1;
ans--;
}
if(y == 0 && last == 1){
ans--;
}
last = y;
}
if(cnt == n){
cout << n;
return;
}
cout << ans;
}
signed main(){
//freopen("time.in", "r", stdin);
//freopen("time.out", "w", stdout);
ios_base::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
int t = 1;
while(t--){
solve();
}
}
