#include<iostream>
#include<vector>
#include<map>
#include<string>
#include<cstring>
#include<bitset>
#include<stack>
#include<queue>
#include<cmath>
#include<algorithm>
#include<cstdio>
#include<numeric>
#include<set>
#include<math.h>
using namespace std;

const int dx[4]={1,0,-1,0};
const int dy[4]={0,1,0,-1};

#define rep(i,x) for(int i=0;i<x;i++)
#define re(i,x,y) for(int i=x;i<y;i++)

long long INF=1e9;
const long long mod=2019;
#define ll long long

double sinc(double x){
    if(x!=0){
        return sin(x)/x;
    }
    else{
        return 1;
    }
}

int main(){
    cin.tie(0); ios::sync_with_stdio(false);
    double n; cin>>n;
    vector<double>x(n),a(n);
    rep(i,n){
        cin>>x[i]>>a[i];
    }
    double sum=0;
    rep(i,n){
        re(j,-100000,100000){
            sum+=(a[i])*sinc(x[i]+j);
        }
    }
    printf("%.16f\n",sum);
    return 0;
}