Lucky numbers

#include <iostream>

using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  string s;
  cin >> s;
  int n=s.size();
  int V=100;
  for(int a=1; a<10; ++a){
    int c=0;
    if(s[0]-'0' != a) ++c;
    int b=a;
    int y=a%7;
    for(int i=1; i<n; ++i){
      if((b+ (s[i]-'0')) %7 >0) ++c;
      b=(14-b)%7;
      y=y*10+b;
      y%=7;
    }
    if(y==0) V=min(V, c);
  }
  cout << V;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED