#include<bits/stdc++.h>
using namespace std;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n;
	cin >> n;
	vector<int> v(n);
	vector<char> c(n);
	for(int i = 0; i < n; i ++)
		cin >> c[i] >> v[i];
	set<double> st;
	map<double, double> to;
	set<double> check;
	for(auto x : v)
	{
		if(check.count(x)) continue;
		if(!check.count(x))
		{
			check.insert(x);
		}
		int TP = 0, FN = 0, FP = 0, TN = 0;
		for(int j = 0; j < n; j ++)
		{
			if(v[j] >= x)
			{
				if(c[j] == '+') TP ++;
				else FP ++;
			}
			else
			{
				if(c[j] == '+') FN ++;
				else TN ++;
			}
		}
		double TPR = TP * 1.0 / (TP + FN);
		double FPR = FP * 1.0 / (TN + FP);
		if(!st.count(FPR))
		{
			st.insert(FPR);
			to[FPR] = TPR;
		}
		else
		{
			to[FPR] = max(to[FPR], TPR);
		}
	}
	
	double now1 = 0;
	double now2 = 0;
	double ans = 0;
	for(auto x : st)
	{
		if(x == 0)
		{
			now2 = to[x];
			continue;
		}
		ans += (x - now1) * now2;
		now1 = x;
		now2 = to[x];
	}
	cout << fixed << setprecision(12) << ans << endl;
	return 0;
}