View Compilation Error

0_0_20330239_32678.c: In function 'main':
0_0_20330239_32678.c:9:5: error: 's' undeclared (first use in this function)
     s = n / 2 * (1 + n);
     ^
0_0_20330239_32678.c:9:5: note: each undeclared identifier is reported only once for each function it appears in