//Metoda trapezului
#include <cstdio>

using namespace std;

int n;
float s;
struct punct
{
    float x, y;
}a[100001];

float arie(punct a, punct b)
{
    return (a.y+b.y)*(a.x-b.x)/2;
}

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

    scanf("%d\n", &n);
    for(int i=0;i<n;i++)
        scanf("%f %f\n", &a[i].x, &a[i].y);

    for(int i=0;i<n-1;i++)
        s+=arie(a[i], a[i+1]);
    s+=arie(a[n-1], a[0]);
    printf("%f", s);
    return 0;
}