#include<cstdio>
#include<utility>
#define mp make_pair
#define nmax 100010
using namespace std;
pair<double,double>v[nmax];
int n,i;
double a,x,y;
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, &y);v[i]=mp(x,y);}
    v[n+1]=v[1];
    for(i=1;i<=n;++i)a+=(v[i].first*v[i+1].second-v[i+1].first*v[i].second);
    if(a<0)a=-a;
    a/=2;
    printf("%lf", a);
    return 0;
}