#include <iostream>
#include <fstream>
using namespace std;
const int MAX=100005;
struct punct{
    double x,y;
}vec[MAX];
int N;
double sol;
int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    fin>>N;
    for(int i=0;i<N;i++)
            fin>>vec[i].x>>vec[i].y;
    vec[N]=vec[0];
    for(int i=0;i<N;i++)
        sol+=(vec[i].x * vec[i+1].y - vec[i+1].x * vec[i].y);
    fout<<sol/2;

    return 0;
}