#include <cstdio>

#define nMax 100010
#define oo 1 << 30

#define abs(a) ((a<0)?-a:a)

using namespace std;

struct puncte{
    double x;
    double y;
}a[nMax];

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




void rez(){
    double s = 0;
    a[n] = a[0];
    for(int i = 0; i < n; ++ i){
        s += a[i].x * a[i + 1].y - a[i].y * a[i + 1].x;
    }

    printf("%lf", s / 2);
}

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

    citire();
    rez();

    return 0;
}