#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <math.h>
#include <time.h>

typedef int32_t i32;
typedef int64_t i64;
typedef double f64;

void *const null=(void*)0;
i64 const llinf=4611686018427387903LL;
i64 const mod=1000000009LL;
f64 const pi=3.141592653589793;
f64 const rad=57.295779513082323;

#define CODE_

#ifndef CODE_
#include "f:/kyopro/code.c"
#define CODE_
#endif

static inline f64 const coordinate_area(i64 const n,f64 const *x,f64 const *y){
    f64 ret=0LL;
    for(i64 i=1LL;i<n;++i) ret+=x[i-1LL]*y[i]-x[i]*y[i-1LL];
    ret+=x[n-1LL]*y[0LL]-x[0LL]*y[n-1LL];
    return fabs(ret)/2.0;
}

i32 main(void){
    i64 n;
    scanf("%lli",&n);
    f64 *x=(f64 *)calloc(n,sizeof(f64));
    f64 *y=(f64 *)calloc(n,sizeof(f64));
    for(i64 i=0LL;i<n;++i) scanf("%lf%lf",&x[i],&y[i]);
    printf("%.1f\n",coordinate_area(n,x,y));
    return 0;
}

