What is wrong with this solution to SPOJ problem BISHOPS - Bishops?

My following C++ solution is getting WA

#include
#include <stdio.h>
#include <string.h>
#include
using namespace std;

int main(){

string N;  
while(cin>>N){

    int len = N.length();

    if (len == 1 && N[0] == '1'){
        printf("1\n");
        continue;
    }

    string res = "";
    int carry = 0;
    for (int i = len - 1; i >= 0; i--){
        int mult = (((N[i] - '0') * 2) + carry);
        carry = mult/10;
        res.push_back((mult%10) + '0');
    }
    if(carry)
        res.push_back (carry + '0');
    reverse (res.begin(), res.end());

    int l = res.length();
    carry = 0;
    string fres;
    int sub = ((res[l - 1] - '0') - 2);
    if (sub < 0){
        sub = sub + 10;
        carry = 1;
        fres.push_back (sub + '0');
        for (int i = l - 2; i >= 0; i--){
            sub = ((res[i] - '0') - carry);
            if(sub < 0){
                sub = sub + 10;
                carry = 1;
            }
            else
                carry = 0;
            fres.push_back(sub + '0');
        }
    }
    else{
        fres.push_back (sub + '0');
        for(int i = l-2; i >= 0; i--)
            fres.push_back(res[i]);
    }

    reverse (fres.begin(), fres.end());
    cout << fres<<endl;
    N = "";
}

return 0;
}

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