#include <bits/stdc++.h>
using namespace std;

int p[1000005], a, b, k;

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

int main() {
    freopen("primes.in","r",stdin);
    freopen("primes.out","w",stdout);

	cin >> a >> b >> k;
	for (int i = 2; i <= b; i++) p[i] = 1;
	for (int i = 2; i <= b; i++)
		if(p[i])
			for (int j = 2 * i; j <= b; j += i)
				p[j] = 0;

	for (int i = 1; i <= b; i++)
		p[i] += p[i - 1];

	int l = 1, r = b - a + 1, m, ret = -1;
	while(l <= r) {
		m = (l + r)/2;
		if(check(m)) ret = m, r = m - 1;
		else l = m + 1;
	}
	cout << ret << endl;
	return 0;
}