CPCRC1C(Sum of Digits) spoj problem solution

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HII GUYS I JUST EXPLAIN HOW I DERIVED THIS FORMULA I JUST MENTION IN MY SEPERATE POST.............HERE IS MY AC C++ SOLUTION IS:::::::::: #include<iostream> #include<cmath> using namespace std; long long sum(long long N) { if(N/10==0) return N*(N+1)/2; int i=0; long long n=N; while(n/10!=0){ i++; n/=10; } int p=pow(10,i); return ((n*45*i*p/10)+ n*(n-1)*p/2+ n*(N%p+1) + sum(N%p)); } int main() { long long a,b; cin>>a>>b; while(a!=-1 || b!=-1) { cout<<sum(b)-sum(a-1)<<endl; cin>>a>>b; } return 0; }