# include <stdio.h>
int f[10];
int main()
{
	int x,y,n,j;
	while(~scanf("%d%d%d",&x,&y,&n))
	{
		
		f[1]=x;
		f[2]=y;
        for(j=3;j<=6;j++)
		{
			f[j]=f[j-1]-f[j-2];
		}
		n%=6;
		if(n==0)
			n=6;
		if(f[n]>=0)
			    printf("%d\n",f[n]%1000000007);
		else
			while(f[n]<0)//注意负数取模  然后就没有然后了~~~~~
				f[n]+=1000000007;
		     	printf("%d\n",f[n]%1000000007);
	}
	return 0;
}

Codeforces Jzzhu and Sequences(循环节)