import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		int[] b = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
			b[i] = sc.nextInt();
		}
		sc.close();

		long[] cnt = new long[4];
		for (int i = 0; i < n; i++) {
			int ai = a[i];
			for (int j = 0; j < 4; j++) {
				if (ai % 2 != 0 || j == 3) {
					cnt[j] += b[i];
					break;
				}
				ai /= 2;
			}
		}

		long ans = cnt[3];
		long min = Math.min(cnt[2], cnt[1]);
		ans += min;
		cnt[2] -= min;
		cnt[1] -= min;
		ans += cnt[2] / 2;
		ans += cnt[1] / 3;
		System.out.println(ans);
	}
}