IOI '05 P2 - Mean Sequence

#include <iostream>
#include <algorithm>

using namespace std;

const long long INF = 1e18;

int main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    int n; cin >> n;
    // no rounding will be needed since all m[i] are even
    long long minX = -INF, maxX = INF;
    long long a = 0; // a +/- x, where x is the first element
    for (int i = 0; i < n; ++i) { // s[i+1]
        long long m; cin >> m; m = 2*m; // convert from mean to sum
        long long b = m - a;
        if (i & 1) minX = max(minX, (a-b)/2); // b + x >= a - x, so x >= (a-b)/2
        else maxX = min(maxX, (b-a)/2); // b - x >= a + x, so x <= (b-a)/2
        a = b;
    }
    cout << max(maxX-minX+1, 0LL) << endl;
    return 0;
}