#include <iostream>
#include <cstdio>
#include <algorithm>
#include <string>
#include <cstring>
#include <map>
#include <set>
#include <vector>
#include <cmath>

using namespace std;

#define f first
#define s second
#define sz size()

const int MAXN = 100 * 100 * 100 + 1;
const double EPS = 0.000000000001;
const int INF = 2 * 1000 * 1000 * 1000;
const int mod = 1000 * 1000 * 1000 + 7;

int a, b, k, v, t = 0, l, r, m, cnt[MAXN];
bool used[MAXN];

bool check (int p) {
    for (int i = a; i <= b - p + 1; i++) {
        if (cnt[i + p - 1] - cnt[i - 1] < k)
            return false;
    }


return true;
}

int main() {
    cin >> a >> b >> k;

    used[0] = used[1] = true;
    for (int i = 2; i < 1001; i++) {
        if (!used[i])
            for (int j = i * i; j <= b; j += i)
                used[j] = true;
    }

    for (int i = 0; i <= b; i++)
        if (!used[i])
            cnt[i] = cnt[i - 1] + 1;
        else
            cnt[i] = cnt[i - 1];

    l = 1;
    r = b - a + 1;

    while (t < 22 && l != r) {
        t++;
        m = (l + r) / 2;
        if (check(m))
            r = m;
            else
                l = m + 1;
    }

    if (check(l)) {
        cout << l;
        return 0;
    }

    if (check(r))
        cout << r;
        else
            cout << -1;


return 0;
}