IOI05_mean.cpp

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

typedef long long ll;

int main() {
  int n; cin >> n;
  vector<ll> a(n + 1), b(n + 1);
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  ll l = LLONG_MIN, r = LLONG_MAX;
  for (int i = 1; i <= n; i++) {
    if (i % 2) {
      b[i] = a[i] - a[i - 1] + b[i - 1];
      r = min(r, b[i]);
    }
    else {
      b[i] = - a[i] + a[i - 1] + b[i - 1];
      l = max(l, b[i]);
    }
  }
  cout << max(r - l + 1, 0ll) << '\n';
}