#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<long long,long long> pll;
#define pb push_back
#define mp make_pair
#define rep(i,n) for(int i=0;i<(n);++i)
constexpr int mod=1000000007;
constexpr int mod1=998244353;
vector<int> dx={0,1,0,-1},dy={-1,0,1,0};
bool inside(int y,int x,int h,int w){
	if(y<h && y>=0 && x<w && x>=0) return true;
	return false;
}





int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	int n;cin >> n;
	n--;
	vector<int> a(n), b(n);
	int sum = 0;
	rep(i,n){
		cin >> a.at(i) >> b.at(i);
		sum += b.at(i);
	}
	ll ans = 0;
	rep(i,n){
		for(int j = (i + 1) % n; j != i; j = (j + 1) % n){
			int k = min(a.at(i), b.at(j));
			a.at(i) -= k;
			b.at(j) -= k;
		}
	}
	rep(i,n) ans += b.at(i);
	ans = sum - ans;
	cout << ans + 1 << endl;
}