Project Euler problem 11

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <stack>
#include <cmath>
#include <map>
#include <ctime>
#define MAXN 111111
#define INF 100000007
using namespace std;
int a[22][22];
int main()
{
for(int i = 0; i < 20; i++)
for(int j = 0; j < 20; j++)
cin >> a[i][j];
int ans = 0;
for(int i = 0; i < 20; i++)
for(int j = 0; j < 20; j++)
{
if(j >= 3) ans = max(ans, a[i][j] * a[i][j - 1] * a[i][j - 2] * a[i][j - 3]);
if(i >= 3) ans = max(ans, a[i][j] * a[i - 1][j] * a[i - 2][j] * a[i - 3][j]);
if(i >= 3 && j >= 3) ans = max(ans, a[i][j] * a[i - 1][j - 1] * a[i - 2][j - 2] * a[i - 3][j - 3]);
if(i <= 16 && j >= 3) ans = max(ans, a[i][j] * a[i + 1][j - 1] * a[i + 2][j - 2] * a[i + 3][j - 3]);
}
cout << ans << endl;
return 0;
}