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

bool primes[1000001];
int dp[1000001];

void init()
{
    int i;

    for (i = 1; i <= 1000000; i++)
    {
        primes[i] = true;
    }
}

void resh()
{
    int i, j;

    primes[1] = false;

    for (i = 2; i <= 1000000; i++)
    {
        if (primes[i])
        {
            for (j = 2 * i; j <= 1000000; j = j + i)
            {
                primes[j] = false;
            }
        }
    }
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    #ifdef ONLINE_JUDGE
    freopen("primes.in", "r", stdin);
    freopen("primes.out", "w", stdout);
    #endif // ONLINE_JUDGE
    init();
    resh();

    int a, b, k, i;

    cin >> a >> b >> k;

    if (primes[a]) dp[a] = 1;
    else dp[a] = 0;
    for (i = a + 1; i <= b; i++)
    {
        dp[i] = dp[i - 1];
        if (primes[i]) dp[i]++;
    }

    for (i = 1; i <= b - a + 1; i++)
    {
        if (dp[b - i + 1] >= k)
        {
            cout << i << '\n';
            return 0;
        }
    }

    cout << -1 << '\n';

    return 0;
}