text: #include <stdio.h>
  #include <math.h>
  
  #define SIZE 20
  
  int grid[SIZE][SIZE] = {
    { 8, 2,22,97,38,15, 0,40, 0,75, 4, 5, 7,78,52,12,50,77,91, 8},
    {49,49,99,40,17,81,18,57,60,87,17,40,98,43,69,48, 4,56,62, 0},
    {81,49,31,73,55,79,14,29,93,71,40,67,53,88,30, 3,49,13,36,65},
    {52,70,95,23, 4,60,11,42,69,24,68,56, 1,32,56,71,37, 2,36,91},
    {22,31,16,71,51,67,63,89,41,92,36,54,22,40,40,28,66,33,13,80},
    {24,47,32,60,99, 3,45, 2,44,75,33,53,78,36,84,20,35,17,12,50},
    {32,98,81,28,64,23,67,10,26,38,40,67,59,54,70,66,18,38,64,70},
    {67,26,20,68, 2,62,12,20,95,63,94,39,63, 8,40,91,66,49,94,21},
    {24,55,58, 5,66,73,99,26,97,17,78,78,96,83,14,88,34,89,63,72},
    {21,36,23, 9,75, 0,76,44,20,45,35,14, 0,61,33,97,34,31,33,95},
    {78,17,53,28,22,75,31,67,15,94, 3,80, 4,62,16,14, 9,53,56,92},
    {16,39, 5,42,96,35,31,47,55,58,88,24, 0,17,54,24,36,29,85,57},
    {86,56, 0,48,35,71,89, 7, 5,44,44,37,44,60,21,58,51,54,17,58},
    {19,80,81,68, 5,94,47,69,28,73,92,13,86,52,17,77, 4,89,55,40},
    { 4,52, 8,83,97,35,99,16, 7,97,57,32,16,26,26,79,33,27,98,66},
    {88,36,68,87,57,62,20,72, 3,46,33,67,46,55,12,32,63,93,53,69},
    { 4,42,16,73,38,25,39,11,24,94,72,18, 8,46,29,32,40,62,76,36},
    {20,69,36,41,72,30,23,88,34,62,99,69,82,67,59,85,74, 4,36,16},
    {20,73,35,29,78,31,90, 1,74,31,49,71,48,86,81,16,23,57, 5,54},
    { 1,70,54,71,83,51,54,69,16,92,33,48,61,43,52, 1,89,19,67,48}
  };
  
  int prod(int x, int y) {
    int product = 1;
    int ty;
    for (int tx = x; tx < SIZE && tx < x + 4; tx++) {
      product *= grid[y][tx];
    }
    int max = product;
    product = 1;
    for (int ty = y; ty < SIZE && ty < y + 4; ty++) {
      product *= grid[ty][x];
    }
    if (product > max) max = product;
    product = 1;
    ty = y;
    for (int tx = x; tx < SIZE && tx < x + 4; tx++) {
      if (ty >= SIZE) {
        break;
      }
      product *= grid[ty][tx];
      ty++;
    }
    if (product > max) max = product;
    product = 1;
    ty = y;
    for (int tx = x; tx < SIZE && tx < x + 4; tx++) {
      if (ty < 0) {
        break;
      }
      product *= grid[ty][tx];
      ty--;
    }
    return (product > max) ? product : max;
  }
  
  int main(void) {
    int p, max = 0;
  
    for (int y = 0; y < SIZE; y++) {
      for (int x = 0; x < SIZE; x++) {
        p = prod(x, y);
        max = (p > max) ? p : max;
      }
    }
    printf("%d\n", max);
  }
  