#include <cstdio>
#include <cmath>
#define N_MAX 100005
using namespace std;

struct {
    double x, y;
} a[N_MAX];

int main() {
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    
    int n; scanf("%d", &n);
    for (int i { 1 }; i <= n; ++i)
        scanf("%lf %lf", &a[i].x, &a[i].y);
    
    a[n + 1] = a[1];
    
    double A { 0 };
    for (int i { 1 }; i <= n; ++i)
        A += a[i].x * a[i + 1].y - a[i + 1].x * a[i].y;
    
    printf("%lf", fabs(A / 2.0));
}