Doubt related to Project euler #8

i was doing project euler #8 it states that

The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.

73167176531330624919225119674426574742355349194934 96983520312774506326239578318016984801869478851843 85861560789112949495459501737958331952853208805511 12540698747158523863050715693290963295227443043557 66896648950445244523161731856403098711121722383113 62229893423380308135336276614282806444486645238749 30358907296290491560440772390713810515859307960866 70172427121883998797908792274921901699720888093776 65727333001053367881220235421809751254540594752243 52584907711670556013604839586446706324415722155397 53697817977846174064955149290862569321978468622482 83972241375657056057490261407972968652414535100474 82166370484403199890008895243450658541227588666881 16427171479924442928230863465674813919123162824586 17866458359124566529476545682848912883142607690042 24219022671055626321111109370544217506941658960408 07198403850962455444362981230987879927244284909188 84580156166097919133875499200524063689912560717606 05886116467109405077541002256983155200055935729725 71636269561882670428252483600823257530420752963450

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

**MY Work/Code **

#include <bits/stdc++.h>
using namespace std;
#define ull unsigned long long int 
int main() {
// your code goes here
string s;
getline(cin,s);
ull max=0;
for(int i=0;i<s.length();i++)
{  
    ull prod=1;
    for(int j=i;j<13+i;j++)
    prod*=(s[j]-'0');
    if(prod>=max)
     max=prod;
}
cout<<max;
return 0;

}

I have never done this type of question in which we have to read input as a string and than operate on it.I don't know if my approach is right or not.Can anyone point me out where i am wrong? P.S:-Sorry,for bad english.