Uva 568 - Just the Facts

#include<stdio.h>
#include<string.h>
int lastdigit(char*buf)
{
const int mod[20]={1,1,2,6,4,2,2,4,2,8,4,4,8,4,6,8,8,6,8,2};
int len=strlen(buf),a[110],i,c,ret=1;
if(len==1)
return mod[buf[0]-'0'];
for(i=0;i<len;i++)
a[i]=buf[len-1-i]-'0';
for(;len;len-=!a[len-1])
{
ret=ret*mod[a[1]%2*10+a[0]]%5;
for(c=0,i=len-1;i>=0;i--)
c=c*10+a[i],a[i]=c/5,c%=5;
}
return ret+ret%2*5;
}
int main(void)
{
char s[1000];
while(scanf("%s",s) == 1)
printf("%5s -> %d/n",s,lastdigit(s));
return 0;
}