#include <stdio.h>

int main()
{
    int n, k;
    double s, p, num, sum;
    double average;
    while(scanf("%d", &n)!=EOF)
    {
        num = 0;
        sum = 0;
        for(int i = 1; i<=n; i++)
        {
            scanf("%lf%lf", &s, &p);
            if(p != -1)
            {
                num += s;
                k = (int)(p);    //将分数进行强制转换成int型
                if(k>=90 && k<=100)
                {
                    sum = sum + s*4.0;
                }
                else if(k>=80 && k<=89)
                {
                    sum = sum + s*3.0;
                }
                else if(k>=70 && k<=79)
                {
                    sum = sum + s*2.0;
                }
                else if(k>=60 && k<=69)
                {
                    sum = sum + s*1.0;
                }
            }
        }
        if(sum == 0.0 || num == 0.0)  //缺考和每门都考但不及格的
            printf("-1\n");
        else
        {
            average = sum*1.0/num;
            printf("%.2lf\n", average);
        }
    }

    return 0;
}