class Solution(object):
    def minPathSum(self, grid):
        if not grid:
            return
        
        m, n = len(grid), len(grid[0])

        for i in range(1, m):
            grid[i][0] = grid[i][0] + grid[i-1][0]

        for j in range(1, n):
            grid[0][j] = grid[0][j] + grid[0][j-1]

        for i in range(1, m):
            for j in range(1,n):
                grid[i][j] = grid[i][j] + min(grid[i-1][j], grid[i][j-1])
                
        return grid[-1][-1]