#include <stdio.h>
int main(){
int a=1,b=2;
int sum;
int n;
if (scanf("%d",&n)!=EOF){
if (n==1){sum=1;}
if (n==2){sum=2;}
if (n>2){
for (int i=3;i<=n;i++){
sum=a+b;
a=b;
b=sum;
}
}
printf("%d",sum);
}
else{
printf("input wrong");
}
return 0;
}
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