import java.io.*;
import java.util.*;
import java.math.*;

class Main {
	
	public static void out (Object o) {
		System.out.println(o);
	}
		
	public static void main (String[] args) throws IOException {
		BufferedReader br = 
			new BufferedReader(new InputStreamReader(System.in));
		
		int n = Integer.parseInt(br.readLine());
		int ans = 0;
		BigDecimal two = BigDecimal.valueOf(2);
		BigDecimal m = BigDecimal.valueOf(1000000007);
		
		for (int i = 0; i < n; i++) {
			String[] line = br.readLine().split(" ");
			BigDecimal c = new BigDecimal(line[0]);
			BigDecimal d = new BigDecimal(line[1]);
			ans += c.divide(two).setScale(0 , RoundingMode.CEILING).multiply(d).remainder(m).intValue();
		}
		
		out(ans);
	}
}