#include <bits/stdc++.h>
#define x first
#define y second

using namespace std;
pair<double, double>p[100005];
int n, i;
double aria;
int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    scanf("%d", &n);
    for(i = 1; i <= n; i++)
        scanf("%lf%lf", &p[i].x, &p[i].y);
    p[++n] = p[1];
    for(i = 1; i < n; i++)
        aria += (p[i].x * p[i + 1].y - p[i + 1].x * p[i].y);
    if(aria < 0.0)
        aria = -aria;
    aria /= 2.0;
    printf("%.10lf", aria);
    return 0;
}