#include <bits/stdc++.h>
using namespace std;
pair<long double,long double> v[100005];
int main(){
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int n;
    scanf("%d",&n);
    long double S=0;
    for(int i=1;i<=n;++i)
        cin>>v[i].first>>v[i].second;
    for(int i=1;i<n;++i)
        S+=v[i].first*v[i+1].second-v[i+1].first*v[i].second;
    S+=v[n].first*v[1].second-v[1].first*v[n].second;
    S=abs(S/2);
    cout<<fixed<<setprecision(5)<<S<<"\n";
    return 0;
}