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

int main()
{int n,i;

fscanf(f,"%d",&n);
for (i=1;i<=n;i++) fscanf(f,"%lf %lf",&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]);
fprintf(g,"%.8lf",arie);
return 0;
}