hdu1202

/**/
#include <cstdio>
#include <cstring>
#include <cmath>
#include <cctype>
#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <vector>
#include <string>
#include <stack>
#include <queue>

typedef long long LL;
using namespace std;

int n;

int judge(double x){
	if(x >= 90){
		return 4;
	}else if(x >= 80){
		return 3;
	}else if(x >= 70){
		return 2;
	}else if(x >= 60){
		return 1;
	}else{
		return 0;
	}
}

int main()
{
	//freopen("in.txt", "r", stdin);
	//freopen("out.txt", "w", stdout);

	while(scanf("%d", &n) == 1){
		double score, p;
		double ans = 0, sum = 0;
		for (int i = 1; i <= n; i++){
			scanf("%lf %lf", &p, &score);
			if(score == -1) continue;
			ans += p * judge(score);
			sum += p;
		}
		if(ans == 0){
			printf("-1\n");
		}else{
			printf("%.2lf\n", ans / sum);
		}
		
	}

	return 0;
}
/**/