#include <cstdio>
#include <algorithm>

using namespace std;

#define inFile "aria.in"
#define outFile "aria.out"
#define MAX_P 100000

FILE *in = fopen(inFile, "r");
FILE *out = fopen(outFile, "w");

struct Point {
    double x;
    double y;
};

Point P[MAX_P + 2];

int main() {
    int i, N;
    long double AREA = 0.0;
    
    fscanf(in, "%d", &N);
    for(i = 1; i <= N; i++) 
        fscanf(in, "%lf %lf", &P[i].x, &P[i].y);
    P[N+1] = P[1];

    for(i = 1; i <= N; i++) 
        AREA += 0.5 * (P[i].x * P[i+1].y - P[i].y * P[i+1].x);
    
    fprintf(out, "%.5lf\n", AREA);
    
    fclose(in);
    fclose(out);
    
    return 0;
}