#include <cstdio>

using namespace std;

int n;

struct pct
{
    double x, y;
}point[100001];

double aria=0;

void citire(int n)
{
    for(int i=0; i<n; i++)
        scanf("%lf %lf", &point[i].x, &point[i].y);
}

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