https://atcoder.jp/contests/agc003/submissions/11407454

b.d

import std.stdio, std.string, std.algorithm, std.array, std.range, std.conv,
    std.typecons, std.math, std.container, std.format, std.numeric;

void main(string[] args)
{
    long n;
    readf("%d\n", &n);

    long ans = 0;
    long before = 0;
    long a;
    foreach (i; 0 .. n)
    {
        readf("%d\n", &a);
        long b = a + before;
        ans += b / 2;
        before = (a == 0) ? 0 : b % 2;
    }

    writeln(ans);
}