View Compilation Error

0_0_17712820_16807.c: In function 'main':
0_0_17712820_16807.c:33:4: error: 'used' undeclared (first use in this function)
    used[n1] = 1;
    ^
0_0_17712820_16807.c:33:4: note: each undeclared identifier is reported only once for each function it appears in