#include<bits/stdc++.h>
using namespace std;
const int MAX_SIZE = 100;
unsigned char field[MAX_SIZE+2][MAX_SIZE+2] = {};
int prefix[MAX_SIZE+2][MAX_SIZE+2] = {};

int sum(int i1, int j1, int i2, int j2)
{
  return prefix[i2][j2] - prefix[i1-1][j2] - prefix[i2][j1-1] + prefix[i1-1][j1-1];
}

void fill_field(int i1, int j1, int i2, int j2)
{
  for (int i = i1-1; i <= i2+1; ++i)
    for (int j = j1-1; j <= j2+1; ++j)
      field[i][j] = 1;
}

int main()
{
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  freopen("INPUT.TXT", "r", stdin);
  int n, m, k;
  cin >> n >> m >> k;

  for (int i=0; i<n+2; ++i)
  {
    field[i][0] = field[i][m+1] = 1;
    prefix[i][0] = i + 1;
  }
  for (int j=0; j<m+2; ++j)
  {
    field[0][j] = field[n+1][j] = 1;
    prefix[0][j] = j + 1;
  }

  for(int i=0; i<k; ++i)
  {
    int i1, i2, j1, j2;
    cin >> i1 >> j1 >> i2 >> j2;
    fill_field(i1, j1, i2, j2);
  }

  for(int i = 1; i < n+1; ++i)
    for(int j = 1; j < m+1; ++j)
        prefix[i][j] = prefix[i-1][j] + prefix[i][j-1] - prefix[i-1][j-1] + field[i][j];

  int square = 0;
  for(int w = m; w > 1; --w)
  {
    bool found = false;
    for(int h = n; h > 1 && !found;  --h)
    {
      for(int i = 1; i + h < n + 2 && !found; ++i)
      {
        for(int j = 1; j + w < m + 2 && !found; ++j)
        {
          if ( sum(i, j, i+h-1, j+w-1) == 0 )
          {
            if (w * h > square)
            {
              square = w * h;
              found = true;
            }
          }
        }
      }
    }
  }

  cout << square;
  return 0;
}