#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <ctime>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <deque>

#define INF (1<<30)
#define mod 666013

using namespace std;

struct coord
{
    double x, y;
}v[100005];
int n, i;
double ar;
int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    scanf("%d", &n);
    for(i=1;i<=n;i++)
        scanf("%lf%lf", &v[i].x, &v[i].y);
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
    for(i=1;i<=n;i++)
        ar=ar+v[i].x*v[i+1].y-v[i].y*v[i+1].x;
    printf("%.5lf", ar/2);
    return 0;
}