L3009 长城

#include <bits/stdc++.h>

using namespace std;

using pii = pair<int, int>;
using ll = long long;

const int N = 1E5 + 10;
ll stk[N];
ll top;
ll x[N], y[N];

bool check(ll a, ll b, ll c) {
    return ((y[a] - y[b]) * (x[a] - x[c]) <= (y[a] - y[c]) * (x[a] - x[b]));
}

int main() {
    int n;
    cin >> n;
    set<ll> res;
    for (int i = 1; i <= n; i++) {
        cin >> x[i] >> y[i];
        if (top >= 1) {
            while (top >= 2 && check(i, stk[top], stk[top - 1])) top--;
            res.insert(stk[top]);
        }
        stk[++top] = i;
    } 

    ll cnt = res.size();
    if (res.count(1)) cnt--;

    cout << cnt << "\n";


    return 0;
}