#include <iostream>
#include <vector>
#include <cmath>
#include <algorithm>
#include <deque>
#include <string>
#include<fstream>
using namespace std;


int main()
{
	long long int n, a, b, sum = 0;
	string s;
	cin >> s;
	fstream file;
	file.open(s, ios::in);
	file >> n;
	vector<long long int> vec;
	for (auto i = 0; i < n; ++i)
	{
		file >> a >> b;
		if (a < b)
			sum += a;
		else
			sum += b;
		vec.push_back(abs(a - b));
	}
	if (sum % 3 == 0)
	{
		cout << sum;
		return 0;
	}
	sort(vec.begin(), vec.end());
	for (auto i = 0; i < int(vec.size()); ++i)
	{
		if (vec[i] % 3 == 1 && sum % 3 == 2)
		{
			sum += vec[i];
			break;
		}
		if (vec[i] % 3 == 2 && sum % 3 == 1)
		{
			sum += vec[i];
			break;
		}
	}
	cout << sum;
	return 0;

}