#include <bits/stdc++.h>
#include <fstream>

#define NMAX 16005

using namespace std;

ifstream f("transport.in");
ofstream g("transport.out");

int n , k , i , c , v[NMAX];

int main()
{
 f >> n >> k;

 for ( i = 1 ; i <= n ; i ++ )
 {
     f >> v[i];
 }

 c = 0;

 for ( i = 1 ; i <= n ; i ++ )
 {
     if ( v[i] >= 3 )
        c += v[i] / 3;
     v[i + 1] += v[i] % 3;

     if ( v[i] < 3 )
        c += 1;
 }

 g << c;

 return 0;
}