C :

#include<stdio.h>
int main()
{
    double fraction;
    while ( scanf ( "%lf", &fraction ) != EOF )
    {
        if(fraction<=59&&fraction>=0) printf("E\n");
        else if(fraction<=69&&fraction>=60) printf("D\n");
        else if(fraction<=79&&fraction>=70) printf("C\n");
        else if(fraction<=89&&fraction>=80) printf("B\n");
        else if(fraction<=100&&fraction>=90) printf("A\n");
        else printf("Score is error!\n");
    }
    return 0;
}

C++ :

#include<bits/stdc++.h>
using namespace std;
int a;
int main()
{
	while(cin>>a)
	{
		switch(a)
		{
			case 90 ...100:cout<<"A"<<endl;break;
			case 80 ...89:cout<<"B"<<endl;break;
			case 70 ...79:cout<<"C"<<endl;break;
			case 60 ...69:cout<<"D"<<endl;break;
			case 0 ...59:cout<<"E"<<endl;break;
			default:cout<<"Score is error!"<<endl;
		}
	}
   return 0;
}