hdu 3792 二分

/*
 * hdu3792.c
 *
 *  Created on: 2011-9-19
 *      Author: bjfuwangzhu
 */

#include<stdio.h>
#include<string.h>
#define nmax 100200
int prime[nmax], flag[nmax];
int plen, nlen;
typedef struct node {
    int i, value;
} node;
node Node[nmax];
void init() {
    memset(flag, -1, sizeof(flag));
    int i, j;
    for (i = 2, plen = 0; i < nmax; i++) {
        if (flag[i]) {
            prime[plen++] = i;
        }
        for (j = 0; (j < plen) && (i * prime[j] < nmax); j++) {
            flag[i * prime[j]] = 0;
            if (i % prime[j] == 0) {
                break;
            }
        }
    }
    Node[0].i = 1, Node[0].value = 0;
    for (i = 1, nlen = 1; i < plen; i++) {
        if (prime[i] - prime[i - 1] == 2) {
            Node[nlen].i = prime[i];
            Node[nlen].value = Node[nlen - 1].value + 1;
            nlen++;
        }
    }
}
void solve(int n) {
    int left, right, mid;
    left = 0, right = nlen;
    while (left <= right) {
        mid = (left + right) / 2;
        if (Node[mid].i > n) {
            right = mid - 1;
        } else {
            left = mid + 1;
        }
    }
    if (Node[left].i > n) {
        left--;
    }
    printf("%d\n", Node[left].value);
}
int main() {
#ifndef ONLINE_JUDGE
    freopen("data.in", "r", stdin);
#endif
    init();
    int n;
    while (~scanf("%d", &n) && (n > 0)) {
        solve(n);
    }
    return 0;
}