#include <stdio.h>
#include <math.h>

#define N_PRIME 700000

int prime_table[N_PRIME];

int main () {
    int p, i, n;
    prime_table[0] = 2;
    prime_table[1] = 3;

    p = 5;
    n = 2;
    for (i = 2; i < N_PRIME; p += n, n = 6 - n) {
        int j, limit = sqrt(p);
        for (j = 2; j < i && prime_table[j] <= limit; ++j)
            if (p % prime_table[j] == 0)
                break;
        if (j >= i || prime_table[j] > limit)
            prime_table[i++] = p;
    }
    printf("%d\n", prime_table[N_PRIME - 1]);
    return 0;
}