Getting WA in MARBLEGF

I have been trying this problem which is supposed to be solved using fenwick trees
The data structure i used is a tree where all the leaves are the array and for every node the value is sum of values of children.
So we can calculate sum of the array between indices i and j in log n time.
But it is saying Wrong Answer.
Please help.
#include
#include <stdio.h>
#include
#include
#include
#define vi vector
#define pi pair<int,int>
#define si stack

typedef long long int ll;
using namespace std;

vector v(2097152,0);

ll sum (ll a,ll b)
{
ll res = 0;
//cout<<a<<" "<<b<<endl;
if (b == (a+1))
{
return v[b]+v[a];
}
while (a < b)
{

	if (a%2 != 0)
	{
		res+=v[a];a++;}
	if(b%2 == 0){res+=v[b];b--;}
	if (a == b)break;
	a/=2;
	b/=2;
//	cout<<a<<" "<<b<<endl;
//	cout<<"REs "<<res<<endl;
}
res+=v[a];
return res;

}
int main ()
{

ll n,q;
cin >>n>>q;
ll var;
for (ll i = 0;i <n;i++)
{
	
	scanf("%lld",&var);
//var = 1000;
	v[i+1048576]=var;
	ll w = (i+1048576)/2;
	while(w >= 1)
	{
		v[w]=v[2*w]+v[2*w+1];
		w/=2;
	}
}
for(ll z =1;z <= q;z++)
{
	char c;
	cin >>c;
	if (c == 'S')
	{
		ll i,j;
		cin>>i>>j;
		//cout<<i<<" "<<j<<endl;
		//for(int i =0;i<5;i++)cout<<v[1048576+i]<<" ";
		cout<<sum(1048576+i,1048576+j)<<endl;
	}
	if (c == 'G')
	{
		ll i,va;
		cin>>i>>va;
			v[i+1048576]+=va;
	ll w = (i+1048576)/2;
	while(w >= 1)
	{
		v[w]=v[2*w]+v[2*w+1];
		w/=2;
	}
	}
	if (c == 'T')
	{
		ll i,va;
		cin>>i>>va;
			v[i+1048576]-=va;
	ll w = (i+1048576)/2;
	while(w >= 1)
	{
		v[w]=v[2*w]+v[2*w+1];
		w/=2;
	}
	}
	
}

}

//