#include <bits/stdc++.h>
using namespace std;
pair<double,double> v[100005];
int main(){
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int n;
    scanf("%d",&n);
    double S=0;
    for(int i=1;i<=n;++i)
        scanf("%lf %lf",&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);
    printf("%.6f\n",S);
    return 0;
}