Easy While Loop Question.

I am trying to get the user to keep inputing something until they enter a sentinel value here is an example of what I want to do:

Code:

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#include <iostream>

using namespace std;

int main()
{
    int MilesDriven;
    float GallonsUsed;
    int TotalGallons;
   
    int TotalMiles;
   
    float MPG;
    int TotalMPG;
   
    while (GallonsUsed != -1)
    {
          TotalGallons = GallonsUsed + GallonsUsed;
          TotalMiles = MilesDriven + MilesDriven;
          TotalMPG = MPG + MPG;
         
          cout <<"How many gallons used : ";
          cin >>GallonsUsed;
         
          cout <<"How many miles driven : ";
          cin >>MilesDriven;
         
          MPG = MilesDriven / GallonsUsed;
         
          cout <<"The miles/gallon for this tank was " <<MPG;
          cout <<endl;
    }
      cout <<"The average miles/gallon was " <<TotalMPG <<endl;
      system("PAUSE");   
      return 0;
}

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It works fine but if the user inputs -1 it continues with the rest of the loop before exiting. I need it to leave the while loop when the user enters -1 for GallonsUsed.

Code:

---

#include <iostream>

using namespace std;

int main()
{
    int MilesDriven;
    float GallonsUsed;
    int TotalGallons;
   
    int TotalMiles;
   
    float MPG;
    int TotalMPG;
   
    while (GallonsUsed != -1)
    {
          TotalGallons = GallonsUsed + GallonsUsed;
          TotalMiles = MilesDriven + MilesDriven;
          TotalMPG = MPG + MPG;
         
          cout <<"How many gallons used : ";
          cin >>GallonsUsed;
         
                  if (GallonsUsed != -1)
                  {

                        cout <<"How many miles driven : ";
                        cin >>MilesDriven;
         
                        MPG = MilesDriven / GallonsUsed;
         
                        cout <<"The miles/gallon for this tank was " <<MPG;
                        cout <<endl;
                  }

                  else cout << endl;
    }
      cout <<"The average miles/gallon was " <<TotalMPG <<endl;
      system("PAUSE");   
      return 0;
}

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