#include<cstdio>
using namespace std;
int N,i; double A;
struct Point {double x,y;} P[100005];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&N);
    for(i=1;i<=N;i++) scanf("%lf%lf",&P[i].x,&P[i].y); P[N+1]=P[1];
    for(i=1;i<=N;i++) A+=(P[i].x*P[i+1].y-P[i].y*P[i+1].x);
    if(A<0) A=-A; A/=2; printf("%lf\n",A);
    return 0;
}