#include<bits/stdc++.h>
using namespace std;
int p[1000000],ans;
int main()
{
    freopen("primes.in","r",stdin);
    freopen("primes.out","w",stdout);
    int a3,ans,a,b,x,l,k,a1=1,a2,br=0;
    bool f;
    scanf("%d%d%d",&a,&b,&k);
    p[1]=1;
    for(int i=4;i<=b;i+=2)p[i]=1;
    for(int i=3;i<=b;i+=2)
    {
        if(p[i]==0)
        {
            for(int j=i*2;j<=b;j+=i)
            {
                p[j]=1;
            }
        }
    }
    a2=b-a+1;
    l=b-a+1;
    br=0;
    for(int i=a;i<=b;++i)
    {
        if(p[i]==0){br++;}
    }
    if(br<k){printf("-1\n");return 0;}
    while(a1<=a2)
    {
        //cout<<a1<<" "<<a2<<endl;
        a3=(a1+a2)/2;
        f=1;
        for(x=1;x<=b-a3+1;++x)
        {
            br=0;
            for(int i=0;i<a3;i++)
            {
                if(p[x+i]==0)br++;
            }
            if(br<k){f=0;break;}
        }
        if(f)
        {
            ans=a3;
            a2=a3-1;
        }
        else
        {
            a1=a3+1;
        }
    }
    printf("%d\n",ans);
return 0;
}