import std.stdio;
import std.algorithm;
import std.string;
import std.range;
import std.array;
import std.conv;
import std.complex;
import std.math;
import std.ascii;
import std.bigint;
import std.container;
import std.typecons;

auto readInts() {
	return array(map!(to!int)(readln().strip().split()));
}
auto readInt() {
	return readInts()[0];
}
auto readLongs() {
	return array(map!(to!long)(readln().strip().split()));
}
auto readLong() {
	return readLongs()[0];
}

const real eps = 1e-10;

void main(){
    auto n = readLong();
    long[] a;
    foreach(i; iota(n)) {
        a ~= readLong();
    }
    long ans;
    foreach(i; iota(n)) {
        ans += a[i] / 2;
        a[i] %= 2;
        if(i < n-1) {
            if(a[i+1] > 0) {
                ans += a[i];
                a[i+1] -= a[i];
            }
        }
    }
    writeln(ans);
}

