C :

int main(int argc, char* argv[])
{
	int num;
	while(~scanf("%d",&num))
	{
	   if(num>=90) printf("A\n");
	   else if(num>=80&&num<90) printf("B\n");
	   else if(num>=70&&num<80) printf("C\n");
	   else if(num>=60&&num<70) printf("D\n");
	   else if(num<60) printf("E\n");
	}
	return 0;
}

C++ :

#include <iostream>

using namespace std;

int main()
{
    double score;
    cin>>score;
    if(score>=90&&score<=100)
        cout<<"A"<<endl;
    else if(score>=80&&score<=89)
        cout<<"B"<<endl;
    else if(score>=70&&score<=79)
        cout<<"C"<<endl;
    else if(score>=60&&score<=69)
        cout<<"D"<<endl;
    else if(score>=0&&score<=59)
        cout<<"E"<<endl;
    return 0;
}