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#include<fstream>
#include<iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
#define maxn 100005
int n,m[2][maxn];
float sol=0;
int main(){
cin>>n;
for(int i=1; i<=n; i++)
cin>>m[0][i]>>m[1][i];
m[0][n+1]=m[0][1];
m[1][n+1]=m[1][1];
for(int i=1; i<=n; i++)
sol+=(m[0][i]*m[1][i+1]-m[0][i+1]*m[1][i]);
cout<<fixed<<setprecision(6)<<sol/2.0;
}