#include <iostream>
#include <fstream>
#include <vector>
#include <cmath>

using namespace std;

struct coord{
double x,y;};

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

vector <coord> v;
int n;

double shoelace(){
    long long p=0;
    for (int i=1;i<=n;++i)
        p+=v[i].x*(v[i-1].y-v[i+1].y);
    p=abs(p);
    return (double)p/2;
}

int main()
{
    double aux1,aux2;
    t.precision(5);
    f>>n;
    v.push_back({0,0});
    for (int i=0;i<n;++i)
        f>>aux1,f>>aux2,v.push_back({aux1,aux2});
    v[0]={v[n].x,v[n].y};
    v.push_back({v[1].x,v[1].y});
    t<<shoelace();
    return 0;
}