This #define here:

#define ull unsigned long long

Should be changed to a typedef instead, like this:

typedef unsigned long long ull;

This line here:

#define ll long long

Should also be changed to this:

typedef long long ll;

In general, if you're using #define to shorten type names, you should be using typedef instead.

You should also drop this line here:

using namespace std;

There are many things that can happen if you have this line, especially if you're working on a large project, and none of them are good. See this Stackoverflow answer for more details.

There's also a reason it's named std and not standard. The prefix std is shorter.

Your inRange function should return a bool, like this:

bool inRange(int x, int y) 
{
    return x >= 0 && a < m && y >= 0 && y < n;
}

This eliminates the need for the if statements.

Instead of defining your own custom infinity (INF), you should be using std::numeric_limits<T>::infinity().

Finally, you don't need to return 0; in main. main is a special function, so the compiler will automatically insert return 0; if it isn't there.

This #define here:

#define ull unsigned long long

Should be changed to a typedef instead, like this:

typedef unsigned long long ull;

This line here:

#define ll long long

Should also be changed to this:

typedef long long ll;

In general, if you're using #define to shorten type names, you should be using typedef instead.

You should also drop this line here:

using namespace std;

There are many things that can happen if you have this line, especially if you're working on a large project, and none of them are good. See this Stackoverflow answer for more details.

There's also a reason it's named std and not standard. The prefix std is shorter.

Your inRange function should return a bool, like this:

bool inRange(int x, int y) 
{
    return x >= 0 && a < m && y >= 0 && y < n;
}

This eliminates the need for the if statements.

Instead of defining your own custom infinity (INF), you should be using std::numeric_limits<T>::infinity().

Finally, you don't need to return 0; in main. main is a special function, so the compiler will automatically insert return 0; if it isn't there.