#include <stdio.h>
#define MAX 1000025
int n, i;
long double x[MAX], y[MAX], A;
int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    scanf("%d",&n);
    for(i=0;i<n;i++) scanf("%Lf%Lf",&x[i],&y[i]);
    x[n]=x[0];
    y[n]=y[0];
    for(i = 0; i < n; i++) A+=(x[i]*y[i+1]-x[i+1]*y[i])/2;
    printf("%.5Lf\n", A);
    return 0;
}