#include <cstdio>
using namespace std;
double x[100002],y[100002],s;
int n,i;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%lf %lf",&x[i],&y[i]);

    x[n+1]=x[1];
    y[n+1]=y[1];
    s=0;

    for(int i=1;i<=n;i++)
        s+=(x[i]*y[i+1]-x[i+1]*y[i]);
    if(s<0)
        s=-s;
    printf("%lf",s/2);
    return 0;
}