#include<stdio.h>
#include<math.h>
#define maxn 100005
using namespace std;

struct point{double x,y;} p[maxn];
int n;
double area;

void read()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++) scanf("%lf%lf",&p[i].x,&p[i].y);
    p[n+1]=p[1];
}

void solve()
{
    for(int i=1;i<=n;i++)
     area+=p[i].x*p[i+1].y-p[i].y*p[i+1].x;
    printf("%lf",area/2);
}

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

    read();
    solve();

    fclose(stdin);
    fclose(stdout);
    return 0;
}