#include <bits/stdc++.h>
using namespace std;int n;long double a,x[100009],y[100009];int main(){ifstream fin("aria.in");ofstream fout("aria.out");fin>>n;for(int i=1;i<=n;++i){fin>>x[i]>>y[i];}x[n+1]=x[1];y[n+1]=y[1];for(int i=1; i<=n; ++i){a+=x[i]*y[i+1]-x[i+1]*y[i];}a=abs(a);return fout<<fixed<<setprecision(10)<<a/2.0,0;}