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#include <cstdio>
#include <cmath>
using namespace std;
FILE*f=fopen("aria.in","r");
FILE*g=fopen("aria.out","w");
struct pct{double x,y;} vec[100005];
int n,sol,i;
int main()
{
fscanf(f,"%d",&n);
for(i=0;i<n;i++)
fscanf(f,"%lf %lf",&vec[i].x,&vec[i].y);
vec[n]=vec[0];
for(i=0;i<n;i++)
sol+=vec[i].x*vec[i+1].y-vec[i].y*vec[i+1].x;
fprintf(g,"%0.9lf\n",fabs(sol/2.0));
return 0;
}