#include <iostream>
#include <cstdio>

using namespace std;
float x[100001],y[100001];
int main()
{FILE *fin=fopen("aria.in","r");
 FILE *fout=fopen("aria.out","w");
double s;
int n,i;
fscanf(fin,"%d",&n);
for(i=1;i<=n;i++)
    {fscanf(fin,"%f",&x[i]);
     fscanf(fin,"%f",&y[i]);
    }
for(i=1,s=0;i<n;i++)
    s+=(x[i]*y[i+1]-x[i+1]*y[i]);
s+=x[n]*y[1]-x[1]*y[n];
if(s<0)
    fprintf(fout,"%f",-s/2);
else
    fprintf(fout,"%f",s/2);
    return 0;
}