红果果的容斥定理。

代码如下：

#include 
     
      #include 
      
       #include 
       
        #include 
        
         using namespace std;int ans, seq[15], N, flag;int M;inline int GCD(int a, int b){    int t;    while (b) {        t = a;        a = b;        b = t % b;    }    return a;}inline int LCM(int a, int b) {    return a / GCD(a, b) * b;}void dfs(int p, int num, int sign){    if (p > 0) {        ans += sign * N / num;    }    for (int i = p + 1; i <= M; ++i) {         dfs(i, LCM(seq[i], num), -sign);    }}int main(){    int i, j;    while (scanf("%d %d", &N, &M) == 2) {        flag = 0;        ans = 0;         for (i = 1, j = 1; i <= M; ++i, ++j) {            scanf("%d", &seq[j]);            if (seq[j] <= 0 || seq[j] >= N) {                --j;            }        }        N -= 1;        M = j - 1;        dfs(0, 1, -1);        printf("%d\n", ans);    }        return 0;}