#include <iostream>
#include <fstream>
#define NMAX 100005
using namespace std;
double x1, y1, x2, y2, firstx, firsty;
long double aria;
int N;

int main()
{
    freopen("aria.in","rt",stdin);
    freopen("aria.out","wt",stdout);
    scanf("%d\n%lf %lf\n", &N, &x1, &y1);
    firstx=x1;
    firsty=y1;
    for(int i=2;i<=N;i++){
        scanf("%lf%lf\n", &x2, &y2);
        aria+= x1*y2 - x2*y1;
        x1=x2;
        y1=y2;
    }
    aria+= x2*firsty - firstx*y2;
    printf("%Lf\n", aria/2);
    return 0;
}