多边形的面积

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <climits>
#include <cstring>
#include <string>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <vector>
#include <list>
#include<functional>
#define mod 1000000007
#define inf 0x3f3f3f3f
#define pi acos(-1.0)
using namespace std;
typedef long long ll;
const int N=205;
const int M=15005;
 
int n,m;
int a[N],dp[N];
int x[N],y[N];
int main(){
    int s=0;
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        scanf("%d%d",&x[i],&y[i]);
    }
    x[n+1]=x[1];y[n+1]=y[1];
    for(int i=1;i<=n;i++){
       s+=x[i]*y[i+1]-x[i+1]*y[i];
    }
    s=abs(s);
    cout<<s/2<<endl;
    return 0;
}