#include <cstdio>
#include <cmath>
#include <fstream>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
long double x[100001],y[100001],arie;
long double ar(long double x1,long double y1,long double x2,long double y2,long double x3,long double y3)
{return 0.5*abs(x1*y2 +x2*y3 +x3*y1 -y2*x3 -y3*x1 -y1*x2);}

int main()
{int n,i;
f>>n;
for (i=1;i<=n;i++) f>>x[i]>>y[i];//fscanf(f,"%llf %llf",&x[i],&y[i]);
for (i=2;i<n;i++) {arie+=ar(x[1],y[1],x[i],y[i],x[i+1],y[i+1]);
                   n++;
                   n--;}
g<<arie;
return 0;
}