Algorithm Analysis

Let's use the inclusion-exclusion principle. The number of attendees at the concert equals $a+b-c$.

Since $a$, $b$, $c$ ≤ $10^{18}$, we should use the long long type.

Implementation of the algorithm

Read the input data. Calculate and print the answer $a+b-c$.

scanf("%lld %lld %lld", &a, &b, &c);
res = a + b - c;
printf("%lld\n", res);