#include <bits/stdc++.h>
using namespace std;

using lli = long long int;

template <class T>ostream &operator<<(ostream &o,const vector<T>&v)
{o<<"{";for(int i=0;i<(int)v.size();i++)o<<(i>0?", ":"")<<v[i];o<<"}";return o;}

template <class T>ostream &operator<<(ostream &o,const pair<T, T>&p)
{o<<"("<<p.first<<", "<<p.second<<")";return o;}

int main(void){
    lli n;
    cin >> n;
    lli si = 0, so = 0;
    for(lli i = 0; i < n-1; i++){
        lli tmp;
        cin >> tmp;
        si+=tmp;
        cin >> tmp;
        so+=tmp;
    }
    lli a, b;
    lli c = 0;
    if(si - so < 0) b = 0;
    else b = si - so;
    a = b + so - si;
    while(a <= so && b <= si){
        c++;
        a++;
        b++;
    }
    cout << c << endl;
    return 0;
}