FERMAT's LITTLE THEOREM

// Generate primes between a and b exclusive and find s=summation(p[i]-1)^p[i] mod p[i]
// http://en.wikipedia.org/wiki/Fermat's_little_theorem

#include<iostream>
#include<vector>
#include<cmath>
int main()
{
    int a,b;
    std::cout<<"Enter a and b: \n";
    std::cin>>a>>b;
    std::vector<int> v;
    for(int i=a+1;i<b;++i)
    {
        bool flag=true;
        for(int j=2;j<=sqrt(i) && flag;++j)
            if(i%j==0)
                flag=false;
        if(flag)
            v.push_back(i);
    }
    long s=0;
    for(std::vector<int>::iterator it=v.begin();it!=v.end();++it)
        s+=(*it)-1;
    std::cout<<"\nSum= "<<s;
}