#include <bits/stdc++.h>
#define MAX 1000001
using namespace std;
bool c[MAX];
int a, b, k, sums[MAX];

void genPrime() {
    for (int i = 2; i <= b; i++) {
        if (!c[i])
            for (int j = i * 2; j <= b; j += i)
                c[j] = true;
    }
    for (int i = 2; i <= b; i++) {
        sums[i] = sums[i - 1];
        if (!c[i])
            sums[i]++;
    }
}

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

int main()
{
    //Watching beginnings of social decay..
    cin >> a >> b >> k;
    genPrime();
    int l = -1, left = 1, right = b - a + 1;
    while (left <= right) {
        int middle = (right + left) / 2;
        if (check(middle)) {
            right = middle - 1;
            l = middle;
        }
        else
            left = middle + 1;
    }
    cout << l;
}