Circular tour

PROBLEM :

Suppose there is a circle. There are n petrol pumps on that circle. You will be given two sets of data.

1. The amount of petrol that every petrol pump has.
2. Distance from that petrol pump to the next petrol pump.

Your task is to complete the function tour which returns an integer denoting the first point from where a truck will be able to complete the circle (The truck will stop at each petrol pump and it has infinite capacity).

Note :  Assume for 1 litre petrol, the truck can go 1 unit of distance.

Input
The first line of input will be the no of test cases . Then T test cases follow . Each Test case contains 2 lines . The first line will contain an integer N denoting the no of petrol pumps and in the next line are N space separated values petrol and distance denoting the amount of petrol every petrol pump has and the distance to next petrol pump respectively .

Output
The output of each test case will be the index of the the first point from where a truck will be able to complete the circle otherwise -1 .

Constraints:
1<=T<=100
1<=N<=50
1<=petrol,distance<=100

Example (To be used only for expected output)
Input
1
4
4 6 6 5 7 3 4 5
Output
1

Explanation:
Above there are 4 petrol pumps with amount of petrol and distance to next petrol pump value pairs as {4, 6}, {6, 5}, {7, 3} and {4, 5}. The first point from where truck can make a circular tour is 2nd petrol pump. Output in this case is 1 (index of 2nd petrol pump).

--------------------------------------------------------------------------------
SIMPLE c++ IMPLEMENTATION :
--------------------------------------------------------------------------------

/*
The structure of petrolPump is
struct petrolPump
{
    int petrol;
    int distance;
};*/

/*You are required to complete this method*/

int tour(petrolPump p[],int n)
{
   int start,end ;
   start=0 ;
   end=1 ;
 
   int CurrPetrol ;
   CurrPetrol=p[start].petrol-p[start].distance ;
 
   while(start!=end||CurrPetrol<0)
   {
       while(CurrPetrol<0)
       {
           CurrPetrol=CurrPetrol-(p[start].petrol-p[start].distance) ;
           start=(start+1)%n ;
         
           if(start==0)
            return -1 ;
       }
       CurrPetrol=CurrPetrol+p[end].petrol-p[end].distance ;
       end=(end+1)%n ;
   }
   return start ;
}

--------------------------------------------------------------------------------

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