Language: C

#include <stdio.h>
int main()
{
	int num, ro, in;
	char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
	char s[10000];
	
	scanf("%d", &num);
	scanf("%s", s);
	
	for(ro = 0; ro < 10000; ro++){
		for(in = 0; in < 26; in++){
			if(s[ro] == alfa[in]){
				s[ro] = alfa[in + num];
			}
		}
	}
	
	printf("%s", s);
	
	return 0;
}

Compiler output:
main.c: In function 'main':
main.c:5:24: error: 'a' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                        ^
main.c:5:24: note: each undeclared identifier is reported only once for each function it appears in
main.c:5:27: error: 'b' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                           ^
main.c:5:30: error: 'c' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                              ^
main.c:5:33: error: 'd' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                 ^
main.c:5:36: error: 'e' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                    ^
main.c:5:39: error: 'f' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                       ^
main.c:5:42: error: 'g' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                          ^
main.c:5:45: error: 'h' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                             ^
main.c:5:48: error: 'i' undeclared (first use in this function); did you mean 'in'?
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                ^
      |                                                in
main.c:5:51: error: 'j' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                   ^
main.c:5:54: error: 'k' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                      ^
main.c:5:57: error: 'l' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                         ^
main.c:5:60: error: 'm' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                            ^
main.c:5:63: error: 'n' undeclared (first use in this function); did you mean 'in'?
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                               ^
      |                                                               in
main.c:5:66: error: 'o' undeclared (first use in this function); did you mean 'ro'?
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                  ^
      |                                                                  ro
main.c:5:69: error: 'p' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                     ^
main.c:5:72: error: 'q' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                        ^
main.c:5:75: error: 'r' undeclared (first use in this function); did you mean 'ro'?
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                           ^
      |                                                                           ro
main.c:5:78: error: 's' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                              ^
main.c:5:81: error: 't' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                                 ^
main.c:5:84: error: 'u' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                                    ^
main.c:5:87: error: 'v' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                                       ^
main.c:5:90: error: 'w' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                                          ^
main.c:5:93: error: 'x' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                                             ^
main.c:5:96: error: 'y' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                                                ^
main.c:5:99: error: 'z' undeclared (first use in this function)
    5 |         char alfa[] = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
      |                                                                                                   ^