#include <cstdio>
#include <cmath>

using namespace std;

int n;
double xi, yi, x, y, X0, Y0;
double Sum;

int main(){
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    scanf("%d", &n);
    scanf("%lf %lf", &xi, &yi);
    X0 = xi;
    Y0 = yi;
    for(int i = 1; i < n; ++i){
        scanf("%lf %lf", &x, &y);
        Sum += (double)(xi * y - x * yi);
        xi = x;
        yi = y;
    }
    Sum += (double)(x * Y0 - X0 * y);
    printf("%lf", fabs(Sum / 2.0));
    return 0;
}