#include <cstdio>

using namespace std;

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n,i;
    double A=0.00,x[100005],y[100005];
    scanf("%d",&n);
    for (i=1;i<=n;i++)
        scanf("%lf %lf",&x[i],&y[i]);
    x[i]=x[1];
    y[i]=y[1];
    for (i=1;i<=n;i++)
        A+=(double)((double)x[i]*y[i+1]-(double)x[i+1]*y[i]);
    A=(double)A/2;
    printf("%.6lf",A);
    fclose(stdin);
    fclose(stdout);
    return 0;
}