#include <fstream>
#include <cstdio>
using namespace std;


ifstream fin("aria.in");
ofstream fout("aria.out");
int xa, xb, xc, ya, yb, yc;
int n, i, j;
double A;
struct punct
{
    int x, y;
};
punct v[100005];

double Aria(int a, int b, int c)
{
    double n;
    n=v[a].x*v[b].y+v[b].x*v[c].y+v[c].x*v[a].y-v[c].x*v[b].y-v[b].x*v[a].y-v[a].x*v[c].y;
    n=n/2.;
    if(n<0)n=-n;

    return n;

}



int main()
{
    freopen("aria.out", "w", stdout);
    fin>>n;
    for(i=1; i<=n; i++)
        fin>>v[i].x>>v[i].y;
    for(j=2; j<=n-1; j++)
        A=A+Aria(1,j,j+1);
    printf("%.6f", A);
    return 0;
}