#include <cstdio>
#include <cmath>
using namespace std;

const int MAX_N = 100002;

int N;
double A;
double v[MAX_N][2];

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][0], &v[i][1]);
    v[N+1][0] = v[1][0], v[N+1][1] = v[1][1];

    for(int i = 1; i <= N; ++i)
        A += v[i][0] * v[i+1][1] - v[i+1][0] * v[i][1];
    A = fabs(A/2);

    printf("%lf\n", A);

    return 0;
}