宝石收集，tarjan

0宝石收集 - 蓝桥云课 (lanqiao.cn)

n=int(input())
s='0'+input()
m=int(input())
mp=[[] for i in range(n+1)]
for i in range(m):
    a,b=map(int,input().split())
    a+=1
    b+=1

    mp[a].append(b)


import sys
sys.setrecursionlimit(100000000)
dfn=[0 for i in range(n+1)]
low=[0 for i in range(n+1)]
cnt=0

stk=[]
instk=[0 for i in range(n+1)]

p=0
scc=[0 for i in range(n+1)]

def tarjan(x):
    global cnt,p
    cnt+=1
    dfn[x]=low[x]=cnt
    stk.append(x)
    instk[x]=1
    for i in mp[x]:
        if dfn[i]==0:
            tarjan(i)
            low[x]=min(low[x],low[i])
        elif instk[i]:
            low[x]=min(low[x],dfn[i])


    if dfn[x]==low[x]:
        p+=1
        y=stk.pop()
        instk[y]=0
        scc[y]=p
        while x!=y:
            y = stk.pop()
            instk[y] = 0
            scc[y] = p
for i in range(1,n+1):
    if dfn[i]==0:tarjan(i)

del instk,low,dfn

ru=[0 for i in range(p+1)]
h=[0 for i in range(p+1)]
l=[0 for i in range(p+1)]
nmp=[[] for i in range(p+1)]
for i in range(1,n+1):
    if s[i]=='0': h[scc[i]]+=1
    else : l[scc[i]]+=1
    for j in mp[i]:
        if scc[i]!=scc[j]:
            ru[scc[j]]+=1
            nmp[scc[i]].append(scc[j])
del mp

def dfs(x,a,b):
    ans=0
    vis=0
    for y in nmp[x]:
        ans=max(dfs(y,a+h[y],b+l[y]),ans)
        vis=1
    if vis==0:return min(a,b)
    else:return ans
ans=0
for i in range(1,p+1):
    if ru[i]==0:
        ans=max(ans,dfs(i,h[i],l[i]))
print(ans)