程序运行错误，求指点，谢谢。。。

#include<iostream>
using namespace std;
int main ()
{
    int score;
    int a;
    cout << "Input score(0~100):"<<endl;
    cin >> score;
    a = score / 1~100;
    switch(a)
    {
        case 100:cout<<"完美"<<endl;break;
        case 90: cout << "优" << endl; break;
        case 80: cout << "良" << endl; break;
        case 70: cout << "中" << endl; break;
        case 60: cout << "及格" << endl; break;
        case 50: cout << "不及格" << endl; break;
    }
    return 0;
}