#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("fast-math")
#include <bits/stdc++.h>
using namespace std;
struct point
{
    double st, dr;
}v[100005];
double arie;
int n;
ifstream fin("aria.in");
ofstream fout("aria.out");
int32_t main(int argc, char * argv[])
{
    fin >> n;
    for(int i = 1; i <= n; ++i)
    {
        fin >> v[i].st >> v[i].dr;
    }
    v[n + 1].st = v[1].st, v[n + 1].dr = v[1].dr;
    for(int i = 1; i <= n; ++i)
    {
        arie = (double)((double)arie + (double)((double)(v[i].st * v[i + 1].dr) - (double)(v[i + 1].st * v[i].dr)));
    }
    arie = (abs)(arie);
    fout << fixed << setprecision(5) << arie / 2;
    return 0;
}