Answer still wrong after many attempts

#1

#include
using namespace std;

int main()
{
int N=0, M=0;
int arr1[1000]={}, arr2[1000]={}, sum_digit[1001]={};
cin>>N;
cin>>M;

//inputing the first digit beforehand so as to determine the sign of the number
int flag=0;
cin>>arr1[0];
if(arr1[0]<0)
{
	flag++;
}
if(flag==0)
{
	for(int i=1; i<N; i++)
	{
		cin>>arr1[i];
	}
}
else
{
	for(int i=1; i<N; i++)
	{
		cin>>arr1[i];
		arr1[i]*=(-1);
	}	
}

//Repeating the same process for the next number
flag=0;
cin>>arr2[0];
if(arr2[0]<0)
{
	flag++;
}
if(flag==0)
{
	for(int j=1; j<M; j++)
	{
		cin>>arr2[j];
	}
}
else
{
	for(int j=1; j<M; j++)
	{
		cin>>arr2[j];
		arr2[j]=-arr2[j];
	}
}

//------------------------main algo for the addition----------------

if(N>=M)
{
	int i=N-1, j=M-1, itr=0, carry=0;
	while((i>=0)&&(j>=0))
	{
		carry=(arr1[i]+arr2[j])/10;
		sum_digit[itr]+=(arr1[i]+arr2[j])%10;
		sum_digit[itr+1]=carry;
		--i;
		--j;
		itr++;
	}

	while(i>=0)
	{
		carry=(sum_digit[itr]+arr1[itr])/10;
		sum_digit[itr]=(sum_digit[itr]+arr1[itr])%10;
		sum_digit[itr+1]=carry;
		--i;
		++itr;
	}
	
	
	for(int ind=itr; ind>=0; ind--)
	{
		if(ind==itr && sum_digit[ind]==0)
        {
            continue;
        }
        cout<<sum_digit[ind]<<", ";
	}
	cout<<"END";
}

else
{
	int i=M-1, j=N-1, itr=0, carry=0;
	while((i>=0)&&(j>=0))
	{
		carry=(arr2[i]+arr1[j])/10;
		sum_digit[itr]+=(arr2[i]+arr1[j])%10;
		sum_digit[itr+1]=carry;
		--i;
		--j;
		itr++;
	}

	while(i>=0)
	{
		
		carry=(sum_digit[itr]+arr2[itr])/10;
		sum_digit[itr]=(sum_digit[itr]+arr2[itr])%10;
		sum_digit[itr+1]=carry;
		--i;
		++itr;
	}

	for(int ind=itr; ind>=0; ind--)
	{
		if(ind==itr && sum_digit[ind]==0)
        {
            continue;
        }
        cout<<sum_digit[ind]<<", ";
	}
	cout<<"END";
}

}