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

using namespace std;

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

int main()
{
    int n;
    in>>n;
    double x1,x2,y1,y2,totalarea=0;
    in>>x1>>y1;
    double fx1=x1,fy1=y1;
    for(int i=2;i<=n;in>>x2>>y2,totalarea+=(y1+y2)*(x1-x2),x1=x2,y1=y2,++i);
    x2=fx1,y2=fy1,totalarea+=(y1+y2)*(x1-x2);
    out<<std::fixed<<std::setprecision(5)<<totalarea/2;
    return 0;
}