#include <cstdio>
#include <algorithm>
#include <cmath>
#include <fstream>

using namespace std;

struct punct{double x,y;};

double Lsegment(punct A, punct B)
{
    return sqrt(1.*abs(A.x-B.x)*abs(A.x-B.x)+abs(A.y-B.y)*abs(A.y-B.y));
}

double Trarie(punct A, punct B, punct C)
{
    double a,b,c,p;
    a=Lsegment(B,C);
    b=Lsegment(A,C);
    c=Lsegment(A,B);
    p=(a+b+c)/2;
    return sqrt(1.*p*(p-a)*(p-b)*(p-c));
}

punct poly[100010];
int n;
double arie;

int main()
{
    freopen("aria.in","r",stdin);
    ifstream fin("aria.in");
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    int i;
    for(i=0;i<n;i++)
        fin >> poly[i].x >> poly[i].y;
    for(i=1;i+1<n;i++)
        arie+=Trarie(poly[0],poly[i],poly[i+1]);
    printf("%g\n",arie);

    return 0;
}