#include<stdio.h>
#include<stdlib.h>
int main(){
    int n,x0,y0,x1,y1,x2,y2;
    double s=0;
    scanf("%d",&n);
    scanf("%d%d",&x0,&y0);
    scanf("%d%d",&x1,&y1);
    n-=2;
    while(n--){
        scanf("%d%d",&x2,&y2);
        s+=abs((x1-x0)*(y2-y0)-(y1-y0)*(x2-x0));
        x1=x2,y1=y2;
    }
    s*=0.5;
    printf("%f\n",s);
    return 0;
}