#include <cstdio>
#include <cmath>

int N;
double sol;
struct {int x; int y;} a[100000];

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

    scanf("%d", &N);

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

    a[N] = a[0];
    for (int i = 0; i < N; ++i) {
        sol += (a[i].x * a[i+1].y - a[i+1].x * a[i].y) / 2.0;
    }

    printf("%lf\n", fabs(sol) );

    return 0;
}