#include <bits/stdc++.h>
#define point pair <double, double>
#define x first
#define y second

FILE *fin  = freopen("aria.in", "r", stdin);
FILE *fout = freopen("aria.out", "w", stdout);

using namespace std;
const int MAX = 1e5 + 2;
int n;
point p[MAX];
double ans;

int main(){
    scanf("%d", &n);
    for(int i = 0; i < n; ++ i)
        scanf("%lf%lf",&p[i].x, &p[i].y);

    p[n] = p[0];
    for(int i = 0; i < n; ++ i)
        ans += p[i].x * p[i + 1].y - p[i + 1].x * p[i].y;
    ans /= 2;
    printf("%.7f\n", ans);
    return 0;
}