View Compilation Error

0_0_22097826_14970.c:5:6: error: variably modified 'a1' at file scope
 char a1[MAXN],a2[MAXN];
      ^
0_0_22097826_14970.c:5:15: error: variably modified 'a2' at file scope
 char a1[MAXN],a2[MAXN];
               ^
0_0_22097826_14970.c:6:6: error: variably modified 'b1' at file scope
 char b1[MAXN],b2[MAXN];
      ^
0_0_22097826_14970.c:6:15: error: variably modified 'b2' at file scope
 char b1[MAXN],b2[MAXN];
               ^
0_0_22097826_14970.c:7:6: error: variably modified 'temp' at file scope
 char temp[MAXN];
      ^
0_0_22097826_14970.c: In function 'input':
0_0_22097826_14970.c:20:5: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
     for(int i=0;i<len;i++) {
     ^
0_0_22097826_14970.c:20:5: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code