import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		
	
		Scanner sc=new Scanner(System.in);
		while(sc.hasNext()) {
			int n=sc.nextInt();
			double sum=0;
			double sum1=0,sum2=0;
			while(n-->0) {
				double a=sc.nextDouble();
				double b=sc.nextDouble();
				
				sum1+=a;
				if (b>=90&&b<=100) {
					sum2+=a*4;
				}
				if (b>=80&&b<90) {
					sum2+=a*3;
				}
				if (b>=70&&b<80) {
					sum2+=a*2;
				}
				if (b>=60&&b<70) {
					sum2+=a*1;
				}
					
				}
			sum=sum2/sum1;
			if (sum1==0) {
				System.out.println("-1");
				}else {
					System.out.println(String.format("%.2f", sum));
			}
			
		}
	}
}