求fibnacci数列的前20项之和

#include "stdio.h"
void main()
{
int n;
int a,b,c;
scanf("%d",&n);
a=0;b=1;
if(n==1) printf("0n");
else if(n==2) printf("1n");
else while(n>2)
{
c=a+b;
a=b;
b=c;
n--;
}
printf("%dn",c);
}