#include <stdio.h>
#include <stdlib.h>
#define MAXN 100001
double x[MAXN],y[MAXN];
inline double a(double x1,double y1,double x2,double y2){
    return (x1*y2-x2*y1)/2;
}
int main(){
    FILE*fi,*fout;
    int i,n;
    double s;
    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]);
    s=0;
    for(i=0;i<n;i++)
       s+=a(x[i],y[i],x[(i+1)%n],y[(i+1)%n]);
    fprintf(fout,"%lf" ,s);
    fclose(fi);
    fclose(fout);
    return 0;
}