# include <iostream>
#include <stdio.h>
using namespace std;
int v[1000001];
int t[1000001];
void res()
{
    int i=2,j;
    v[1]=1;
    while(i<1000000)
        {
            if(v[i]){i++;continue;}
            j=i+i;
            while(j<=1000000)
            {
                v[j]=1;
                j+=i;
            }
            i++;
        }
}
int main()
{
freopen("primes.in","r",stdin);
freopen("primes.out","w",stdout);

int a,b,k;
res();
//cout<<v[2]<<" "<<v[3]<<" "<<v[4]<<endl;
    cin>>a>>b>>k;
int i;
for(i=a;i<=b;i++)
{
    t[i]=t[i-1];
    if(!v[i])t[i]++;
}
if(t[b]<k){cout<<-1<<endl;return 0;}
//cout<<t[2]<<" "<<t[3]<<" "<<t[4]<<endl;
int x,y;
x=1;
y=b-a+1;
int z;
bool fl=1;
while(x<y)
{
    fl=0;
    z=(x+y)/2;
    for(i=a;i<=b-z+1;i++)
    {
        if(t[i+z-1]-t[i-1]<k){fl=1;break;}
    }
   // cout<<z<<" "<<fl<<endl;
    if(!fl)
    {
        y=z;
    }
    else
    {
        x=z+1;
    }
}
cout<<x<<endl;
return 0;
}