#include <bits/stdc++.h>

using namespace std;

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

#define fi first
#define se second
#define db double

#define MAX_N 100000

pair <db, db> v[MAX_N + 2];

int main()  {
    int n;
    fscanf(fin, "%d", &n);
    for(int i = 1;i <= n;i++)  {
        fscanf(fin, "%lf%lf", &v[i].fi, &v[i].se);
    }
    v[n + 1].fi = v[1].fi;
    v[n + 1].se = v[1].se;
    db aria = 0.0;
    for(int i = 1;i <= n;i++)  {
        aria += (v[i].fi * v[i + 1].se - v[i].se * v[i + 1].fi);
    }
    fprintf(fout, "%lf", aria / 2);
    fclose(fin);
    fclose(fout);
    return 0;
}