scanf("%c",&a[100]) continuous input in c

Question

//it accepts input but skips first and last char of sentence anyone explan it clearly.

#include<stdio.h>
#include <stdlib.h>

int main()
{
    int i=0,j=0;
    char S[100];
    scanf("%c",&S[100]);
    fgets(S,sizeof S,stdin);
    while(S[i])
    {
        printf("%c",S[i]);
        i++;
    }
}