count every square，if it next to another, count - 2

int count = 0;

for (int i = 0; i < grid.length; i++){

for (int j = 0; j < grid[i].length; j++){

if (grid[i][j] == 1){

count += 4;

if (j != 0 && grid[i][j - 1] == 1) count -= 2;

if (i != 0 && grid[i - 1][j] == 1) count -= 2;

}

}

}

return count;