Gym 313459E

#include<bits/stdc++.h>

using namespace std;

#define gc c=getchar()
#define r(x) read(x)
#define ll long long

template<typename T>
inline void read(T&x){
    x=0;T k=1;char gc;
    while(!isdigit(c)){if(c=='-')k=-1;gc;}
    while(isdigit(c)){x=x*10+c-'0';gc;}x*=k;
}

const int N=1e6+7;

bool mark[N];
int prime[N],tot;
bool vis[N];

int main(){
    // freopen(".in","r",stdin);
    // freopen(".out","w",stdout);
    ll s,e,k;r(s),r(e),r(k);
    int n=e-s+1;
    ll ans=0;
    for(int i=2,cnt=0;cnt<k;++i){
        if(!mark[i]){
            prime[++tot]=i;
            for(int x=(i-s%i)%i;cnt<k&&x<n;x+=i){
                if(!vis[x]){
                    vis[x]=1;
                    ans+=i;
                    ++cnt;
                }
            }
        }
        for(int j=1,tmp;j<tot&&(tmp=i*prime[j])<N;++j){
            mark[tmp]=1;
            if(i%prime[j]==0)break;
        }
    }
    printf("%lld\n",ans);
    return 0;
}