You can see the original problem here:

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In the 2020 grid below, four numbers along a diagonal line have been marked in red.

08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08
49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00
81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65
52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 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 03 45 02 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 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21
24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72
21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95
78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92
16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57
86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58
19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40
04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66
88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69
04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36
20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16
20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54
01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48

The product of these numbers is 26 63 78 14 = 1788696.

What is the greatest product of four adjacent numbers in the same direction (up, down, left, right, or diagonally) in the 2020 grid?

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My Solution

#include <stdio.h>

int main(){
  int mat[20][20];
  int i,j,max,x,y,right,down,diag;
  max=0;
  for (i=0;i<20;i++)
    for (j=0;j<20;j++){
      scanf("%d",&x);
      mat[i][j]=x;
      }

  for (i=0;i<20;i++)
    for (j=0;j<17;j++){
    right = mat[i][j]*mat[i][j+1]*mat[i][j+2]*mat[i][j+3];
    if (right>max)
      max=right;
    }

  for (i=0;i<17;i++)
    for (j=0;j<20;j++){
    down = mat[i][j]*mat[i+1][j]*mat[i+2][j]*mat[i+3][j];
    if (down>max)
      max=down;
    }

  for (i=0;i<17;i++)
    for (j=0;j<17;j++){
    diag = mat[i][j]*mat[i+1][j+1]*mat[i+2][j+2]*mat[i+3][j+3];
    if (diag>max)
      max=diag;
    }

  for (i=0;i<17;i++)
    for (j=3;j<20;j++){
    diag = mat[i][j]*mat[i+1][j-1]*mat[i+2][j-2]*mat[i+3][j-3];
    if (diag>max)
      max=diag;
    }

  printf("%dn",max);

  return 0;
}