>>321 (1)
#include <stdio.h>
#define LEN 100
int main(void) {
  char data1[LEN+1], data2[LEN+1], data3[LEN*2+1];
  int n, len;

  printf("S1="); fgets(data1, LEN+1, stdin); len = strlen(data1)-1; data1[len] = '\0';
  printf("S2="); fgets(data2, LEN+1, stdin); data2[strlen(data2)-1] = '\0';
  printf("n="); scanf("%d", &n);
  if(len < n) n = len;
  strncpy(data3, data1, n);
  data3[n] = '\0';
  strcat(data3, data2);
  strcat(data3, &data1[n]);
  puts(data3);
  return 0;
}