// dfmt off
T lread(T=long)(){return readln.chomp.to!T;}T[] lreads(T=long)(long n){return iota(n).map!((_)=>lread!T).array;}
T[] aryread(T=long)(){return readln.split.to!(T[]);}void arywrite(T)(T a){a.map!text.join(' ').writeln;}
void scan(L...)(ref L A){auto l=readln.split;foreach(i,T;L){A[i]=l[i].to!T;}}alias sread=()=>readln.chomp();
void dprint(L...)(lazy L A){debug{auto l=new string[](L.length);static foreach(i,a;A)l[i]=a.text;arywrite(l);}}
static immutable MOD=10^^9+7;alias PQueue(T,alias l="b<a")=BinaryHeap!(Array!T,l);import std, core.bitop;
// dfmt on

void main()
{
    long N = lread();
    auto A = lreads(N);
    auto dp = new long[][](2, N + 1);
    dp[0][] = dp[1][] = long.min;
    dp[0][0] = 0;

    foreach (i; 0 .. N)
    {
        dprint(dp[0][i], dp[1][i]);
        if (dp[1][i] != long.min)
        {
            long a = A[i] + 1;
            long r = (a < 2) ? 0 : a % 2;
            dp[r][i + 1] = dp[r][i + 1].max(dp[1][i] + a / 2);
        }
        if (dp[1][i] != long.min)
        {
            long a = A[i];
            long r = (a < 2) ? 0 : a % 2;
            dp[r][i + 1] = dp[r][i + 1].max(dp[1][i] + a / 2);
        }
        if (dp[0][i] != long.min)
        {
            long a = A[i];
            long r = (a < 2) ? 0 : a % 2;
            dp[r][i + 1] = dp[r][i + 1].max(dp[0][i] + a / 2);
        }
    }
    writeln(max(dp[0][N], dp[1][N]));
}
