#include <fstream>
#include <cmath>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
double A, xx, yy, a, b, c, d, e, f, g, v, n, i, x[100005], y[100005];
double aria(int ax, int ay, int bx, int by, int cx, int cy);
int main()
{
    cin>>n;
    cin>>xx>>yy;
    for(int i=2; i<=n; i++)
        cin>>x[i]>>y[i];

    for(int i=2; i<n; i++)
        A+=aria(xx, yy, x[i], y[i], x[i+1], y[i+1]);

    A*=100000;
    f=A-int(A);
    A-=f;
    A/=100000;

    if(A!=int(A))
        cout<<A;
    else
        cout<<A<<".0";
    return 0;
}

double aria(int ax, int ay, int bx, int by, int cx, int cy)
{
    ax-=cx;
    ay-=cy;
    bx-=cx;
    by-=cy;
    return (double)(ax*by-ay*bx)/2;
}