CodeForces - 630F Selection of Personnel （组合数学）

CodeForces - 630F Selection of Personnel Time Limit: 500MS   Memory Limit: 65536KB   64bit IO Format: %I64d & %I64u Submit Status Description One company of IT City decided to create a group of innovative developments consisting from 5 to 7 people and hire new employees for it. After placing an advertisment the company received n resumes. Now the HR department has to evaluate each possible group composition and select one of them. Your task is to count the number of variants of group composition to evaluate. Input The only line of the input contains one integer n (7 ≤ n ≤ 777) — the number of potential employees that sent resumes. Output Output one integer — the number of different variants of group composition. Sample Input Input 7 Output 29 Source Experimental Educational Round: VolBIT Formulas Blitz #include<stdio.h> #include<math.h> #include<string.h> #include<algorithm> #define INF 0x3f3f3f3f #define ll long long #define N 10010 #define M 1000000007 using namespace std; ll C(ll n,ll x) { ll i,j,s=1; for(i=n,j=1;i>n-x;i--,j++) s*=i,s/=j; return s; } int main() { int n; while(scanf("%d",&n)!=EOF) { ll sum=C(n,5)+C(n,6)+C(n,7); printf("%lld\n",sum); } return 0; }