#include <cstdio>
using namespace std;

struct punct
{
    float x, y;
}a[100010];

int n, i, j;
float p, s;
int main (){
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    scanf("%d", &n);
    for (i=1; i<=n; i++)
        scanf("%f%f", &a[i].x, &a[i].y);

//    for (i=1; i<=n; i++) printf("%f %f\n", a[i].x, a[i].y);
//    printf("\n");

    a[n+1]=a[1];
    for (i=1; i<=n; i++){
        p=(a[i].x*a[i+1].y-a[i+1].x*a[i].y)/2;
        s+=p;
    }
    printf("%f", s);
    return 0;
}