#include <cstdio>
#define nmx 100002
using namespace std;

struct punct{
    double x,y;
} v[nmx];

int n;

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

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

    v[0] = v[n];

    double sum = 0;
    for(int i = 1; i <= n; ++i)
        sum += (v[i-1].x-v[i].x)*(v[i-1].y+v[i].y);
    sum /= 2;

    printf("%.5lf", sum);

    return 0;
}