ACM HDU 1005 Number Sequence

#include <cstdio>
int f(int a,int b,int n)
{
    int i,nn,n1,n2;
    if(n==1||n==2)return 1;
    a%=7;
    b%=7;
    n%=49;
    n1=n2=1;
    for(i=2; i<n; i++)
    {
        nn=(a*n2+b*n1)%7;
        n1=n2;
        n2=nn;
    }
    return nn;
}
int main(void)
{
    int a,b,n;
    while(scanf("%d%d%d",&a,&b,&n),a)
        printf("%d\n",f(a,b,n));
    return 0;
}