public class Birth{
public static void main(String args[]){
int a=1;//can give birth
int b=0;//wait for 1 year
int c=0;//wait for 2 years
int d=0;//wait for 3 years
int y=0;//year
int f=0;//changable
for(y=1;y<=20;y++){
f=a;a+=b;b=c;c=d;d=f;}
System.out.println(a+b+c+d);}
}