#include<cstdio>
int i,n;
double a,x[100007],y[100007];
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)
        a+=x[i]*y[i+1]-x[i+1]*y[i];
    printf("%lf",a/2.0);
    return 0;
}