#include <stdio.h>
#include <math.h>

int main() {
    int n;
    scanf("%d", &n);
    int x[n + 2], y[n + 2];
    for (int i = 1; i <= n; ++i) {
        scanf("%d", &x[i]);
        scanf("%d", &y[i]);
    }
    x[n + 1] = x[1];
    y[n + 1] = y[1];
    float arie = 0;
    for (int i = 1; i <= n; ++i) {
        arie += (double)((fabs(x[i] * y[i + 1] - x[i + 1] * y[i])))/2; 
    }
    printf("%.5f", arie);
    return 0;
}