#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream in("aria.in");
ofstream out("aria.out");

inline double abs(double x){
    return x > 0 ? x : -x ;
}

int main()
{
    int n;
    in>>n;
    int x1,x2,y1,y2;
    in>>x1>>y1;
    double carea,totalarea=0;
    for(int i=2;i<=n;++i){
        in>>x2>>y2;
        carea=((abs(y1)+abs(y2))*(x2-x1))/2;
        totalarea+=carea;
    }
    out<<std::fixed<<std::setprecision(5)<<totalarea;
    return 0;
}