Codeforces Round #411 (Div. 2), problem: (D) Minimum number of steps Solution In C/C++

#include<bits/stdc++.h>
using namespace std;
int c,d,i,n,m,k,x,j=1000000007;
string s;
main(){
cin>>s;
for(i=s.size()-1;i>=0;i–){
if(s[i]==’b’)c++;else{
k+=c;c*=2;k%=j;c%=j;
}
}
cout<<k;
}