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    "# Problem 15:\n",
    " \n",
    "### [Euler Project #15](https://projecteuler.net/problem=15)\n",
    " \n",
    "### Starting in the top left corner of a 2×2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.\n",
    "\n",
    "### How many such routes are there through a 20×20 grid?\n",
    "\n",
    "---"
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    "### Let's try to map the example cases, by assigning a binary value to RIGHT and DOWN moves.\n",
    "\n",
    " 0 = R <br>\n",
    " 1 = D\n",
    "\n",
    " 0 0 1 1 <br>\n",
    " 0 1 0 1 <br>\n",
    " 0 1 1 0 <br>\n",
    " 1 0 0 1 <br>\n",
    " 1 0 1 0 <br>\n",
    " 1 1 0 0 <br>\n",
    "\n",
    "### A couple things that shake out of this,\n",
    "1. The total length of a sequence of moves is the equal to Length + Width of the grid.\n",
    "2. Each unique sequence of moves has a twin which is a mirror across the diagonal.\n",
    "3. Each valid sequence has an equal number of RIGHT and DOWN moves.\n",
    "\n",
    "### I'm sure there is some combinatorial mathematics that describes how to do this analytically, but I'd rather practice programming a loop, than researching an elegant solution. \n",
    "\n",
    "---"
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