In this Ciel and A-B problem why is the solution wrong.thanks in advance

`#include <iostream>

using namespace std;
int main() {
int a,b;
cin>>a>>b;
int c=a-b;
if(c%10>0)
cout<<c-1;
else
cout<<c+1;
return 0;
}`

when a-b == 1 your answer would be 0.but the answer should be a positive integer, that is why giving wrong answer

Thanks a lot I miss these critical test values everytime