I do not know how let this C program run correctly

Question

#include <stdio.h>

char s[80];

int i, j,n;

int main() {

   printf("Type in a word: "); 

   scanf("%s",s);

   printf("Type in a number: ");

   scanf("%d",&n);

   i = 0;

   j = strlen -1;

   while (i < j) {

printf("%c",s[i]);

i = i + n;

   }

}