#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
#define DUMP(x) cout << #x << " = " << (x) << endl;
#define FOR(i, m, n) for (ll i = m; i < n; i++)
#define IFOR(i, m, n) for (ll i = n - 1; i >= m; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define FOREACH(x, a) for (auto&(x) : (a))
#define ALL(v) (v).begin(), (v).end()
#define SZ(x) ll(x.size())

int main() {
  ll n;
  cin >> n;
  vector<ll> a(n,0), b(n,0);
  REP(i, n - 1) {
    ll x, y;
    cin >> x >> y;
    a[i+1] = a[i] + x;
    b[i+1] = b[i] + y;
  }
  ll ub = b[n-1];
  ll lb = 0;
  FOR(i,1,n){
    lb = max(lb, a[i]-a[i-1]+b[i]-b[i-1]-a[n-1]);
  }
  cout << max(ub - (lb-1), 0LL) << endl;
}