为什么不开O2就会RE?

#include<bits/stdc++.h>
using namespace std;

const int N=1e6+5,M=2e6+5;
int n,m,s,t,ans,dep[N],now[M];
int first[N],nex[M],to[M],w[M],tot=1;

void add(int x,int y,int z)
{
	nex[++tot]=first[x];
	first[x]=tot;
	to[tot]=y;
	w[tot]=z;
}

int dfs(int x,int flow)
{
	if(x==t) return flow;
	int rest=flow,i,k;
	for(int i=now[x];i&&rest;i=nex[i])
	{
		int y=to[i];
		if(w[i] && dep[y]==dep[x]+1)
		{
			k=dfs(y,min(rest,w[i]));
			if(!k) dep[y]=0;
			w[i]-=k;
			w[i^1]+=k;
			rest-=k;
		}
	}
	now[x]=i;
	return flow-rest;
}

bool bfs()
{
	memset(dep,0,sizeof(dep));
	queue<int>a;
	a.push(s);dep[s]=1;now[s]=first[s];
	while(!a.empty())
	{
		int x=a.front();a.pop();
		for(int i=first[x];i;i=nex[i])
		{
			int y=to[i];
			if(w[i] && dep[y]==0)
			{
				a.push(y);
				now[y]=first[y];
				dep[y]=dep[x]+1;
				if(y==t) return 1;
			}
		}
	}
	return 0;
}

int main()
{
	scanf("%d%d%d%d",&n,&m,&s,&t);
	s+=n;
	for(int i=1;i<=n;i++)
	{
		add(i,i+n,1);
		add(i+n,i,0);
	}
	int x,y;
	for(int i=1;i<=m;i++)
	{
		scanf("%d%d",&x,&y);
		add(y+n,x,0x3f3f3f3f);
		add(x,y+n,0);
		add(x+n,y,0x3f3f3f3f);
		add(y,x+n,0);
	}
	int flow=0;
	while(bfs())
		while(flow=dfs(s,0x3f3f3f3f)) ans+=flow;
		
	cout<<ans;
	return 0;
}