1115

 1 #include <iostream>
 2 #include <stdio.h>
 3 using namespace std;
 4 int main()
 5 {
 6     int i,n,a,b,c,sum;
 7     a=3,b=4,c=5;
 8     cin>>n;
 9     if(n==1) sum=3;
10       else if(n==2) sum=4;
11       else if(n==3) sum=5;
12     else
13         for(i=4;i<=n;i++)
14 {
15     sum=a+b+c;
16     a=b;
17     b=c;
18     c=sum;
19 }
20     cout<<sum<<endl;
21     return 0;
22 }