Calculate the difference between this temperature and the previous one

A Simple Question of Chemistry

#include <fstream>
#include <iostream>
using namespace std;
//Calculate the difference between this temperature and the previous one.
int main()
{
    //ifstream cin("a_simple_question_of_chemistry.txt");
    double a,b;
    cin>>a;
    while(cin>>b)
    {   
        if(b==999)
        {   
            cout<<"End of Output"<<endl;
            break;
        }   
        printf("%.2f\n",b-a);
        a=b;
    }   
    return 0;
}