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

// Moi cap la 2 la bai co gia tri chenh nhau <= 1. Cac cap khong duoc dung chung la bai.
// Cac gia tri xep thanh chuoi lien tiep. Duyet gia tri tang dan, mang "du" la so la bai
// cua gia tri truoc chua ghep duoc (toi da 1 la, vi phan con lai duoc ghep noi bo).
// Uu tien ghep cheo truoc (moi cap cheo dung 1 la cua moi gia tri), roi ghep noi bo.

// Chi co mot cach xu ly cho bai nay (khong co subtask), nen chi viet 1 sub.
// Dau vao: A_i la so la bai co gia tri i.
long long sub1(const map<long long, long long> &cnt) {
    long long ans = 0, carry = 0, prev = -10;
    for (auto &kv : cnt) {
        long long v = kv.first, c = kv.second;
        if (v != prev + 1) carry = 0;    // hai gia tri khong lien tiep thi khong ghep cheo duoc
        long long m = min(carry, c);     // ghep cheo voi la du cua gia tri truoc
        ans += m;
        c -= m;
        ans += c / 2;                    // ghep noi bo gia tri nay
        carry = c % 2;                   // con 1 la le thi giu lai de ghep voi gia tri sau
        prev = v;
    }
    return ans;
}

int main() {
    // Dong dau: N. N dong tiep theo: A_i = so la bai ghi gia tri i (i = 1..N)
    int n;
    cin >> n;
    map<long long, long long> cnt;
    for (int i = 1; i <= n; i++) {
        long long a;
        cin >> a;
        if (a > 0) cnt[i] = a;
    }
    cout << sub1(cnt) << endl;
    return 0;
}