#include <bits/stdc++.h>
using namespace std;
const int N = 1e4 + 15, M = 2e5 + 10;

vector<int> edge[N];
int h[N], e[M], ne[M], idx, p[N];
int in[N];
int n, m;
void add(int a, int b) {
    e[idx] = b, ne[idx] = h[a], h[a] = idx ++ ;
    in[b]++;
}

int dfn[N], low[N], tot = 0;
stack<int> stk;
bool st[N];
int sz[N], scc[N], color;

void tarjan(int u) {
    dfn[u] = low[u] = ++tot;
    stk.push(u); st[u] = true;
    for (int v : edge[u]) {
        if (!dfn[v]) tarjan(v), low[u] = min(low[u], low[v]);
        else if (st[v]) low[u] = min(low[u], dfn[v]);
    }
    if (dfn[u] == low[u]) {
        ++color;
        while (stk.top() != u) {
            scc[stk.top()] = color;
            st[stk.top()] = false; stk.pop();
        }
        scc[stk.top()] = color;
        st[stk.top()] = false; stk.pop();
    }
}

queue<int> q;
int dp[N];
void topsort() {
    for (int i = 1; i <= color; i++) {
        dp[i] = sz[i];
        if (in[i] == 0) q.push(i);
    }
    while (q.size()) {
        int u = q.front(); q.pop();
        for (int i = h[u]; ~i; i = ne[i]) {
            int v = e[i];
            dp[v] = max(dp[v], dp[u] + sz[v]); in[v]--;
            if (in[v] == 0) q.push(v);
        }
    }
    int ans = 0;
    for (int i = 1; i <= color; i++) ans = max(ans, dp[i]);
    printf("%d\n", ans);
}

int main() {
    memset(h, -1, sizeof h);
    n = 2, m = 1;
    for (int i = 1; i <= n; i++) scanf("%d", &p[i]);
    while (m--) {
        int a = 1, b = 2;
        edge[a].push_back(b);
    }
    for (int i = 1; i <= n; i++)
        if (!dfn[i]) tarjan(i);
    for (int i = 1; i <= n; i++) {
        sz[scc[i]] += p[i];
        for (int j : edge[i])
            if (scc[i] != scc[j]) add(scc[i], scc[j]);
    }
    topsort();
    return 0;
}

转自AcWing@Conan15