#include<cstdio>
using namespace std;
int n,i;
double aria;
double x[100002],y[100002];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++)
        scanf("%lf%lf",&x[i],&y[i]);

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

    for(i=1;i<=n;i++)
        aria+=x[i]*y[i+1]-x[i+1]*y[i];
    aria/=2.0;
    printf("%lf",aria);
    return 0;
}