#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#define popcount __builtin_popcount
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;
double sinc(double x){
	if(abs(x)<1e-7) return 1.0;
	else return sin(x)/x;
}
int main()
{
	int n; cin>>n;
	double ans=0;
	double a[1010], x[1010];
	for(int i=0; i<n; i++){
		cin>>x[i]>>a[i];
		x[i]-=floor(x[i]);
	}
	for(int i=0; i<n; i++){
		for(int j=-100000; j<=100000; j++){
			if(abs(j)==1 && x[i]<1e-10) continue;
			ans-=a[i]*(1/(x[i]+j+1)/(x[i]+j-1))*sinc(x[i]+j)/(2*sin(0.5)*sin(0.5));
		}
	}
	printf("%.10lf\n", ans);
	return 0;
}