#include <fstream>
#include <math.h>

using namespace std;

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

struct coord{
    int x, y;
};

int n;
coord p[1000];

float area(){
    float a = 0;

    for(int i=1; i+1<n; i++){
        int x1 = p[i].x - p[0].x ;
        int x2 = p[i+1].x - p[0].x ;

        int y1 = p[i].y - p[0].y ;
        int y2 = p[i+1].y - p[0].y;

        int c = x1*y2 - x2*y1;
        a += c;
    }

    if(a > 0)return a/2;
    else if(a == 0)return 0;
    else return -a/2;
}

int main()
{
    fin>>n;

    for(int i=0; i<n; i++){
        fin>>p[i].x>>p[i].y;
    }

    fout<<area();
}