getting TLE from mouse maze in c++ - c++

Question

I keep getting TLE in this question.I have tried to use scanf printf instead of cin cout but it didnt work.Then,I tried another question which has same
description,the only different is input N which means test case change from 1000 to 10^6.However, I got all AC there.I just cant figure out why.
following is the question
Description
Write a program that simulates a mouse in a maze. The program must count the steps taken by the mouse from the starting point to the final point.
The maze type is shown in following figure:
S$###
$$#$$
$$$##
##$$F
it consists of S (starting point), #(walls), $(road) and F (final point).
In above case, it needs 7 steps from S to F as following figure,
S$###
$$#$$
$$$##
##$$F
and the mouse can move in the four directions: up, down, left, right. There may be more than one way to reach final point, the program only need to print the least steps.
If there is no way from S to F, then print -1.
Input
The first line has an integer N(1<=N<=1000), which means the number of test cases.
For each case, the first line has two integers. The first and second integers R and C (3<=R, C<=500) represent the numbers of rows and columns of the maze, respectively. The total number of elements in the maze is thus R x C.
The following R lines, each containing C characters, specify the elements of the maze.
Output
Print out the least steps for each case, and there is a new line character at the end of each line.
Sample Input
3
4 5
S$###
$$#$$
$$$##
##$$F
4 5
S$$##
#$$$#
#$#$#
#$$$F
4 5
##S$#
$##$$
$$$##
#F###
Sample Output
7
7
-1
following is my code
#include <iostream>
#include <climits>
using namespace std;
int least_step;
void maze(char a[501][501],int,int,int,int,int);
#define wall '#'
int main(){
int ncase;
cin>>ncase;
char a[501][501];
while(ncase--){
int num1,num2;
cin>>num1>>num2;
least_step = INT_MAX;
int si,sj;
for(int i=0;i<num1;i++){
for(int j=0;j<num2;j++){
cin >> a[i][j];
if(a[i][j]=='S'){
si = i;
sj = j;
}
}
}
maze(a,si,sj,num1,num2,0);
if(least_step==INT_MAX){
least_step = -1;
}
cout<<least_step<<endl;
}
return 0;
}
void maze(char a[501][501],int i,int j,int num1,int num2,int step){
if(a[i][j] == 'F'){
if(step<least_step){
least_step = step;
}
return;
}
a[i][j] = wall;
if(i+1<num1&&a[i+1][j] != wall){
maze(a,i+1,j,num1,num2,step+1);
}
if(j+1<num2&&a[i][j+1] != wall){
maze(a,i,j+1,num1,num2,step+1);
}
if(i-1>=0&&a[i-1][j] != wall){
maze(a,i-1,j,num1,num2,step+1);
}
if(j-1>=0&&a[i][j-1] != wall){
maze(a,i,j-1,num1,num2,step+1);
}
a[i][j] = '$';
return;
}