#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<bitset>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<deque>
#include<list>
#include<iomanip>
#include<cmath>
#include<cstring>
#include<functional>
using namespace std;

#define repr(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(i, n) repr(i, 0, n)
#define INF 2e9
#define MOD 1000000007
//#define MOD 998244353
#define LINF (long long)4e18
#define jck 3.141592

using ll = long long;
using Pi = pair<int,int>;
using Pl = pair<ll,ll>;


int main(){
    int N; cin >> N;
    vector<int> A(N-1),B(N-1);
    int sum1 = 0;
    rep(i,N-1){
        cin >> A[i] >> B[i];
        sum1 += B[i];
    }
    rep(i,N-1){
        for(int j = N-2; j >= 0; j--){
            if(i != j){
                int d = min(A[j],B[i]);
                B[i] -= d;
                A[j] -= d;
            }
        }
    }
    int sum2 = 0;
    rep(i,N-1){
        sum2 += B[i];
    }
    int ans = sum1-sum2+1;
    cout << ans << endl;
}