#include <stdio.h>
#include <stdlib.h>
#define MAXN 100001
float x[MAXN],y[MAXN];
inline float a(int x1,int y1,int x2,int y2){
    return (x1*y2-x2*y1)/2;
}
int main(){
    FILE*fi,*fout;
    int i,n;
    float s;
    fi=fopen("aria.in" ,"r");
    fout=fopen("aria.out" ,"w");
    fscanf(fi,"%d" ,&n);
    for(i=0;i<n;i++)
       fscanf(fi,"%f%f" ,&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,"%f" ,s);
    fclose(fi);
    fclose(fout);
    return 0;
}