为什么会编译错误

#include <bits/stdc++.h>
using namespace std;
int n,m,dx[] = {0,1,0,-1,0},dy[] = {0,0,1,0,-1},ans = 0;
char a[21][21];
bool vis[21][21];
int main(){
	scanf("%d%d",&n,&m);
	int sx,sy;
	for(int i = 1;i<=m;i++)
		for(int j = 1;j<=n;j++){
			cin>>a[i][j];
			vis[i][j] = true;
			if(a[i][j] == '@')sx = i,sy = j;
		}
	dfs(sx,sy);
	printf("%d",ans);
	return 0;
}
void dfs(int x,int y){
	vis[x][y] = false;
	ans++;
	for(int i = 1;i<=4;i++){
		int tx = x+dx[i],ty = y+dy[i];
		if(x>=1 && x<=m && y>=1 && y<=n && a[x][y] != '#' && !vis[x][y])dfs(tx,ty);
	}
}