#include <stdio.h>
#define MAXN 100000
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, "%lf%lf", &x[i], &y[i]);
    }
    aria=0;
    for(i=0; i<n; i++){
        aria+=(x[i]*y[(i+1)%n]-x[(i+1)%n]*y[i]);
    }
    fprintf(fout, "%.6lf", aria/2.0);
    fclose(fin);
    fclose(fout);
    return 0;
}