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| #include <bits/stdc++.h>
#define int long long
#define double long double
#define pii pair<int, int>
#define N 1005
#define x first
#define y second
#define IOS ios::sync_with_stdio(0),cin.tie(0)
using namespace std;
int n;
struct pt{
int x,y;
bool operator == (pt b){
if(x == b.x && y == b.y)return true;
return false;
}
pt operator - (pt b){return {x - b.x , y - b.y};}
pt operator + (pt b){return {x + b.x , y + b.y};}
int operator ^ (pt b){return (x * b.y - y * b.x);}
int operator * (pt b){return (x * b.x + y * b.y);}
}pot[N];
int solve(){
int ans = 0;
for(int i=0;i<n;i++)ans += (pot[i] ^ pot[i+1]);
return ans;
}
signed main(){
IOS;
cin>>n;
for(int i=0;i<n;i++)cin>>pot[i].x>>pot[i].y;
pot[n] = pot[0];
cout<<abs(solve())<<"\n";
}
|