#include <stdio.h>
#define MAXN 100001
long double x[MAXN], y[MAXN];
int main(){
    int n, i;
    double aria;
    FILE *fin, *fout;
    fin=fopen("aria.in", "r");
    fout=fopen("aria.out", "w");
    fscanf(fin, "%d", &n);
    for(i=0; i<n; i++){
        fscanf(fin, "%llf%llf", &x[i], &y[i]);
    }
    aria=0;
    x[n]=x[0];
    y[n]=y[0];
    for(i=0; i<n; i++){
        aria+=(x[i]*y[i+1]-x[i+1]*y[i]);
    }
    fprintf(fout, "%.10llf", aria/2.0);
    fclose(fin);
    fclose(fout);
    return 0;
}