#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <iomanip>
using namespace std;

double dot_product(vector<double>x, vector<double> y)
{
    double sum=0;
    for(int i=0; i<x.size(); ++i)
    {
        sum+=x[i]*y[i];
    }
    return sum;
}

double sum(vector<double> x, int p=1)
{   double sum=0;
    for(int i=0; i<x.size(); i++)
        sum+=pow(x[i], p);
    return sum;
}
double mean(vector<double> x)
{   double total=sum(x);
    return total/x.size();
}

double linear_regression(vector<double> x, vector<double>y, double t)
{   double n=double(x.size());
    double sumX=sum(x,1);
    double sumX2=sum(x,2);
    double sumY=sum(y,1);
    double meanX=mean(x);
    double meanY=mean(y);
    
    double b=(n*dot_product(x, y)-sumX*sumY)/(n*sumX2-sumX*sumX);
    double a=meanY-b*meanX;
    
    return a+b*t;
    
}

int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    int a=5;
    vector<double> math(a);
    vector<double> stat(a);
    double m,s;    
    for(int i=0; i<a; ++i)
    {
        cin>>m>>s;
        math.push_back(m);
        stat.push_back(s);
    }
    double math_grade=80;
    double stat_grade=linear_regression(math,stat,math_grade);
    
    cout<<fixed<<setprecision(3);
    cout<<stat_grade<<endl;
    return 0;
}

}