#include<bits/stdc++.h>
#define pld pair<long double,long double>
#define N 100005
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n;
long double ans;
int main()
{
    ios::sync_with_stdio(false);
    fin>>n;
    pld v[n+1];
    for(int i=0;i<n;++i)
        fin>>v[i].first>>v[i].second;
    v[n]=v[0];
    for(int i=0;i<n;++i)
        ans += v[i].first * v[i+1].second - v[i+1].first * v[i].second;
    fout<<fixed<<setprecision(6)<<fabs(ans/2);
}