#include <cstdio>

using namespace std;

int n,i;
double x[100005],y[100005];
double a;

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]);
    a=x[n]*y[1]-x[1]*y[n];
    for(i=2;i<=n;i++)
        a+=x[i-1]*y[i]-x[i]*y[i-1];
    a/=2;
    printf("%.6lf\n",a);
    return 0;
}