#include<stdio.h>
#define N 100000
#include<math.h>
double x[N+1],y[N+1];
int main(){
    FILE *fin,*fout;
    fin=fopen("aria.in","r");
    fout=fopen("aria.out","w");
    int n,i;
    fscanf(fin,"%d",&n);
    for(i=0;i<n;i++)
        fscanf(fin,"%lf%lf",&x[i],&y[i]);
    double ans=0;
    x[n]=x[0],y[n]=y[0];
    for(i=0;i<n;i++){
        ans+=(x[i]*y[i+1]-x[i+1]*y[i]);
    }
    fprintf(fout,"%.6lf",ans/2.0);
    return 0;
}