改了3天了求大佬帮助QwQ

#include<bits/stdc++.h>
using namespace std;
int n,m,s,t,ans;
vector< vector<int> > a;
bool visit[1000005];
int dfs(int now,int bs){
    if(visit[now])
        return -1;
    if(now==t)
        return bs;
    int ans=-1;
    for(int i=0;i<a[now].size();i++){
        int tmp=dfs(a[now][i],bs+1);
        if(ans==-1)
            ans=tmp;
        else if(tmp>-1)
            ans=min(ans,tmp);
    }
    return ans;
}
signed main(){
    memset(visit,false,sizeof(visit));
    cin>>m>>n;
    a.resize(m);
    for(int i=1,ta,tb;i<=n;i++){
        cin>>ta>>tb;
        a[ta].push_back(tb);
    }
    cin>>s>>t;
    ans=dfs(s,0)-1;
    cout<<(ans==-2?-1:ans);
    return 0;
}