100% speed Python [No imports][Simple to understand / commented!]
Code: \n\ndef countStu(stu,sand):\n    while sand:\n        if sand[0] in stu:\n            stu.remove(sand[0])\n            sand.pop(0)\n        else:break\n  
Code: \n```\ndef countStu(stu,sand):\n    while sand:\n        if sand[0] in stu:\n            stu.remove(sand[0])\n            sand.pop(0)\n        else:break\n    return len(sand)\n```\n\nCommented Code:\n```\ndef countStu(stu,sand):\n\t# while there are still elements in sandwich list\n    while sand:\n\t\t# check if the first element of sand is in the student list\n        if sand[0] in stu:\n\t\t\t# remove the element from the student list\n            stu.remove(sand[0])\n\t\t\t# remove the first element in the sandwich list\n            sand.pop(0)\n\t\t# if the first element of sand isn\'t in the student list break the while loop\n        else:break\n\t# after removing all the sandwiches that have a student to eat them, the length of sand is the\n\t# amount of uneaten sandwiches\n    return len(sand)\n```\n
['Python', 'Python3']