SPOJ - NDIV Cant find the error

Question

I have implemented sieve approach and it runs correctly for my test cases. I used a counter rather than a flag for all numbers and the times the numbers been accessed. When i run it on SPOJ it gives wrong answer. Here is the link http://www.spoj.com/problems/NDIV/

#include<iostream>
#include<cmath>
#include<vector>
using namespace std;
int main(){
vector<long int> vec;
long int sum=0,n,i,j,k,a,b,c;
double root;
cin>>a>>b>>n;
root = sqrt(b);

for(i=1;i<=b;i++) vec.push_back(1);

for(i=2;i<=root;i++){
    k=i;
    c=0;
    for(j=2;j<=b && c<=b;j++){
        c=k*j;

        if(c<=b){
            vec[c-1]++;
        }
    }

}

for(i=a;i<=b;i++){  
    if(sqrt(i)-floor(sqrt(i))!=0){

        if(vec[i-1]*2 == n) {
            sum++;
        }

    }
    else {

        if(vec[i-1]*2-1 == n) {

            sum++;
        }
    }

}
cout<<sum;
}