Simple one-line Calculator in C++

#include <iostream>
#include <sstream>
using namespace std;
int main()
{
    cout<<"Enter equation like this 5+8 then press enter"<<endl<<endl;
    int x , y , i=0 , n , digits , max;
    string input , num1="" , num2="";
    cin>>input;
    digits=input.size()-1;
    again:
    i=i+1;
    if(input[i]=='+')
    {
        n=i;
        while(n>0)
        {
            num1=num1+input[i-n];
            n=n-1;
        }
        digits=digits-num1.size();
        max=digits;
        while(digits>0)
        {
            num2=num2+input[i+1+(max-digits)];
            digits=digits-1;
        }
    }
    else
    {
        goto again;
    }
    stringstream a(num1);
    a>>x;
    stringstream b(num2);
    b>>y;
    cout<<" = "<<x+y;
}