Create a function def island_perimeter(grid): that returns the perimeter of the island described in grid

`def island_perimeter(grid): perimeter = 0 for i in range(len(grid)): for j in range(len(grid[i])): if grid[i][j]: perimeter += 4 if i > 0 and grid[i-1][j]: perimeter -= 2 if j > 0 and grid[i][j-1]: perimeter -= 2 return perimeter`