#include <cstdio>
#define MAXN 100000
double x[MAXN],y[MAXN];
int main(){
    FILE*fi,*fout;
    int i,n;
    double sum;
    fi=fopen("aria.in" ,"r");
    fout=fopen("aria.out" ,"w");
    fscanf(fi,"%d" ,&n);
    for(i=0;i<n;i++)
       fscanf(fi,"%lf%lf" ,&x[i],&y[i]);
    sum=0;
    for(i=0;i<n;i++)
      sum=sum+(x[i]*y[(i+1)%n]-x[(i+1)%n]*y[i]);
    fprintf(fout,"%.6lf" ,sum/2);
    fclose(fi);
    fclose(fout);
    return 0;
}