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

typedef long long ll;
typedef long double ld;
typedef vector < long long > vll;
typedef pair < long long, long long > pll;
typedef pair < int, int > pii;
typedef vector < int > vii;

#define csl ios_base::sync_with_stdio(false); cin.tie(NULL)
#define l(x) (((x) << 1) | 1)
#define r(x) ((l(x)) + 1)
#define mp make_pair
#define fst first
#define snd second

ll t, n, u, v, m, q, r, ql, qr, k, l, s, x, y, w;
const int N = 1e5 + 500;
const long long mod = 1e9 + 7;
const long long INF = 1LL << 57LL;
const bool JUDGE = false;

int main(){
    csl;
    if (JUDGE) {
        freopen("in.txt", "r", stdin);
        freopen("out.txt", "w", stdout);
    }
    cin >> n;
    ll next;
    ll prev = - INF;
    ll cura = -INF;
    ll curb = INF;
    ll ans = INF;
    for (int i = 0; i < n; ++i) {
        cin >> next;
        curb = min(curb, next);
        if (next < prev) ans = 0;
        ll tempa, tempb;
        tempa = 2 * next - curb;
        tempb = 2 * next - cura;
        cura = tempa;
        curb = tempb;
        if (cura > curb) ans = 0;
        prev = next;
    }
    if (ans) cout << curb - cura + 1 << '\n';
    else cout << ans << '\n';
    return 0;
}