147C

#include <algorithm>
#include <iostream>
#include <cstdio>
#include <vector>
#include <map>
#include <string>
#include <cstring>
#include <cmath>
#include <set>
#include <list>
#include <queue>
using namespace std;

#define LL long long
#define V vector
#define VI V<int>
#define VII V<VI>
#define FOR(t,l,r) for (int t=l; t<r; t++)
#define FORL(t,l,r) for (LL t=l; t<r; t++)
const double mth_pi=2*acos(0.);
#define pi mth_pi
#define inf (1<<23)
#define eps 1e-5
#define pb push_back
#define mp make_pair
#define sz size()
#define mii map<int,int>
#define pii pair<int,int>
#define x first
#define y second

const int NN=1234567;
int lp[NN+1], pr[NN], cc=0;
void resh () {
    FOR(i,2,NN+1) {
        if (!lp[i]) {
            lp[i]=i, pr[cc++]=i;}
        int t=i*pr[0];
        for (int j=0; j<cc && pr[j]<=lp[i] && t<=NN; ++j, t=i*pr[j])
            lp[t]=pr[j];} }

int main () {
    int a, b, k, kk, t;
    cin >>a>>b>>k;
    VI p; resh();

    for (t=a; t<=b; ++t) if (lp[t]==t) p.pb(t);
    int n=p.sz, l=0;
    if (n<k) {cout <<"-1\n"; return 0;}
    for (t=0; t<n-k; ++t) l=max(l,p[t+k]-p[t]);

    kk=0;
    for (t=a; t<=a+l-1; ++t) kk+=(lp[t]==t);
    while (t<=b && kk<k) kk+=(lp[t]==t), t++, l++;
    if (l>b-a+1) {cout <<"-1\n"; return 0;}

    kk=0;
    for (t=b; t>=b-l+1; --t) kk+=(lp[t]==t);
    while (t>=a && kk<k) kk+=(lp[t]==t), t--, l++;
    if (l>b-a+1) {cout <<"-1\n"; return 0;}

    cout <<l<<endl;
return 0;
}