do while循环问题

#include<iostream>
using namespace std;
int main()
{
    int a=0;
    do
    {
    cout<<"请输入学生成绩:";
    cin>>a;
    if(a>100){cout<<"你的输入有误"<<endl;}
    }
    while(a>100);
    {
        if(a>=80){cout<<"此分数为A级"<<endl;}
        else if(a>=60){cout<<"此分数为B级"<<endl;}
        else if(a>=40){cout<<"此分数为C级"<<endl;}
        else if(a>=20){cout<<"此分数为D级"<<endl;}
        else cout<<"此分数为E级";
    }
    system ("pause");
    return 0;
}