input:
42 hfgf=
output:
2829124

input:
1 fgh=
output:
1

input:
-1 fgh=
output:
1

#include <stdio.h>

long f(size_t x) { return x * x; }
long g(size_t x) { return x + 1; }
long h(size_t x) { return x - 1; }

long doMath(char func, size_t x) {
    long value;

    switch(func) {
        case 'f':
            value = f(x);
            break;
        case 'g':
            value = g(x);
            break;
        case 'h':
            value = h(x);
            break;
    }

    return value;
}

int main() {
    char input[50];

    printf("Input: "); fgets(input, 50, stdin);

    int _i = 0, _j = 0;
    int num = 0; char funcs[10];
    long value;

    while (input[_i] != '=') {
        int c = input[_i];

        /* 0-9 */
        if (c >= 48 && c <= 57) {
            num *= 10; num += c - 48;
        }
        /* lowercase a-z */
        if (c >= 97 && c <= 122) {
            funcs[_j++] = (char)c;
        }

        _i++;
    }

    value = num;

    for (_j = 0; funcs[_j] != ''; _j++) {
        value = doMath(funcs[_j], value);
    }

    printf("Output: %lin", value);
}