count the number of substrings that start and end with 1

       
        
        Given a binary string S. The task is to count the number of substrings that starts and end with 1.
        Note: The starting and the ending 1s should be different. 

        Example 1:

        Input:
        S = "10101"
        Output: 3
        Explanation: The 3 substrings are "101","10101" and "101".

        Example 2:

        Input: 
        S = "100"
        Output: 0
        Explanation: No substring that starts and ends with 1
        
        

        Code
               
        int countSubstr (string s)
        {
            
            int i,j,n;
            n=s.size();
            int c=0;
            for(i=0;i<n;i++)
            {
                if(s[i]=='1')
                 c++;
            }
            int k=c*(c-1);
            return k/2;
             
            
        }